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PHYSICAL  LABORATORY 
HANDBOOK 


GEORGE  A.  HOADLEY 


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PHYSICAL  LABORATORY 
HANDBOOK 


BY 
GEORGE    A.    HOADLEY,    C.E.,  Sc.D. 

PROFESSOR    OF     PHYSICS    IN     SWARTHMORE    COLLEGE 


NEW   YORK  •:•  CINCINNATI  •:•  CHICAGO 

AMERICAN     BOOK     COMPANY 


COPYRIGHT,  1909,  BY 
GEORGE   A.    HOADLEY. 

ENTERED  AT  STATIONERS'  HALL,  LONDON. 


H.    PHYS.    LAB.    BOOK. 

W.  P.     I 


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PREFACE 

THIS  book  is  designed  to  provide  a  suitable  number  of 
experiments  for  a  course  in  Physics,  including  those  needed 
to  meet  the  entrance  requirements  of  colleges  and  universities. 
It  will  be  found  especially  convenient  in  the  schools  that  use 
the  author's  "Elements  of  Physics"  as  a  text-book,  because 
the  order  of  subjects  is  the  same;  but  it  can  be  used  also 
in  connection  with  other  texts. 

An  attempt  has  been  made  to  avoid  a  too  detailed  descrip- 
tion of  apparatus  or  method  of  experimentation  and  rather 
to  give  suggestions  from  which  the  pupil,  with  the  assistance 
of  his  instructor,  can  work  out  such  details  as  the  apparatus 
at  hand  will  require.  A  pupil  generally  learns  more  from  an 
experiment  if  he  devises  his  own  apparatus  than  he  does  if  he 
uses  apparatus  provided  for  him. 

Tables  of  physical  constants  are  omitted  for  the  reason  that 
they  are  to  be  found  in  most  text-books  of  Physics  and  because 
every  laboratory  should  be  equipped  with  at  least  one  copy 
of  the  "  Smithsonian  Physical  Tables."  This  book  is  not 
expensive,  and  it  furnishes  all  the  tables  that  are  likely  to  be 
needed  for  reference.  Hering's  "Conversion  Tables"  is  also 
a  desirable  reference  book. 

The  form  of  notebook  to  be  used  by  the  pupil  in  which  to 
record  the  work  done  in  the  laboratory,  is  left  to  the  discretion 
of  the  teacher.  Colleges  and  universities  generally  require 
a  certificate  for  the  notebook,  in  a  form  which  can  be  obtained 
from  them. 


CONTENTS 


PAGH 

SUGGESTIONS  FOR  EXPERIMENTING .7 

HINTS  FOR  SIMPLE  LABORATORY  PROCESSES 8 

EXPERIMENTS 

1.  Measurements  of  Lengths,  Areas,  and  Volumes          ...       13 

2.  To  find  the  Surface  and  Volume  of  a  Cylinder  and  of  a  Sphere  15 

3.  To  find  the  Diameter  of  a  Wire 17 

4.  To  find  the  Mass  per  Unit  Volume  or  the  Density  of  a  Body      .  19 

5.  To  study  the  Relation  between  the  Force  acting  upon  an  Elastic 

Body  and  the  Distortion  produced  by  it  ....       21 

6.  To  find  the  Breaking  Strength  of  a  Wire 24 

7.  To   determine  the  Conditions  of  Equilibrium  between  Three 

Concurrent  Forces  acting  in  the  Same  Plane.     The  Parallelo- 
gram of  Forces .         .26 

8.  Equilibrium  of  Three  Parallel  Forces  in  the  Same  Plane    .         .       28 

9.  Laws  of  the  Pendulum 30 

10.  The  Moments  of  Forces -   .         .32 

11.  To  determine  whether  the  Point  of  Application  of  the  Weight 

of  a  Lever  is  in  the  Same  Position  as  its  Center  of  Gravity    .  33 

12.  The  Inclined  Plane 34 

13.  Friction 35 

14.  The  Principle  of  Archimedes.  — The  Lifting  Effect  of  a  Liquid  37 
lo.    To  determine  the  Specific  Gravity  of  Solids  heavier  than  Water  38 

16.  To  determine  the  Specific  Gravity  of  a  Body  lighter  than  Water      39 

17.  To  determine  the  Specific  Gravity  of  Liquids      ....       40 

18.  To  determine  the  Specific  Gravity  of  Air    .         .  .         .44 

19.  To  find  the  Relation  between  the  Pressure  upon  a  Given  Mass 

of  Air  and  the  Resultant  Volume.  — Boyle's  Law  ...       45 

20.  To  measure  Gas  Pressure  by  the  Use  of  a  Manometer         .         .       47 

4 


CONTENTS  5 

EXPERIMENTS  PAGE 

21.  To  determine  the  Velocity  of  Sound  in  the  Laboratory  and  to 

measure  the  Wave  Length  of  Sound 48 

22.  To  determine  the  Number  of  Vibrations  of  a  Tuning  Fork         .  49 

23.  Testing  the  Fixed  Points  of  a  Thermometer        .         .         .         .51 

24.  To  find  the  Coefficient  of  Linear  Expansion  of  Solids          .         .  53 

25.  To  determine  the  Dew-point  and  Relative  Humidity  ...  54 

26.  To  find  the  Law  of  Heat  Exchange  by  the  Method  of  Mixtures  56 

27.  To  find  the  Specific  Heat  of  a  Solid 56 

28.  To  determine  the  Heat  of  Fusion  of  Ice 58 

29.  To  determine  the  Heat  of  Vaporization  of  Water       ...  59 

30.  To  find  the  Pressure  of  Saturated  Ether  Vapor  at  Different 

Temperatures  ..........  61 

31.  Lines  of  Force  in  a  Magnetic  Field 63 

32.  Mapping  Lines  of  Force  in  a  Magnetic  Field       ....  65 

33.  To  make  a  Permanent  Magnet    .......  66 

34.  Study  of  a  Single-fluid  Galvanic  Cell  .         .        .         .         .        .68 

35.  The  E.  M.  F.  of  a  Single-fluid  Cell,  and  the  Electromotive  Series  69 

36.  To  study  a  Two-fluid  Cell  and  to  compare  with  it  a  Dry  Cell     .  70 

37.  Arrangement  of  Cells  to  produce  the  Greatest  Current  in  a 

Given  Circuit 71 

38.  Effect  of  an  Electric  Current  upon  the  Temperature  of  a  Con- 

ductor        71 

39.  Lines  of  Force  about  a  Current-carrying  Conductor   ...  72 

40.  To  study  an  Electro-magnet 73 

41.  To  make  and  study  a  Lifting  Magnet          .....  74 

42.  Electroplating 77 

43.  To  study  the  Resistance  of  a  Conductor 77 

44.  Effect  of  Temperature  on  the  Resistance  of  a  Conductor.     Study 

of  the  Change  of  Resistance  of  an  Iron  Wire  due  to  Change 

of  Temperature         .....         ....  78 

45.  To  measure  the  Resistance  of  a  Conductor  by  the  Fall  of  Poten- 

tial Method 79 

46.  Distribution  of  Current  over  the  Branches  of  a  Divided  Circuit  81 


6  CONTENTS 

EXPERIMENTS  PAGB 

47.  To  measure  the  Resistance  of  a  Conductor  by  the  Wheatstone 

Bridge      .         .         . , .  82 

48.  The  Internal  Resistance  of  a  Cell  ••    \     <  .     *  .       -j -.••;-••    .  .  83 

49.  Effect  of  Cutting  Lines  of  Force  with  a  Conductor     .  *j$  *  .  85 

50.  To  study  a  Dynamo     .  ^ -.  '•  .,.'••  "    . "'  ?  .     •    .        ;    •";  .  88 

51.  To  study  an  Electric  Motor         .        .      -  .        .        . ;      v  .  89 

52.  Study  of  an  Electric  Bell     .     •-,'     -.     >   \:       .'       .   .  -  :v  -^    .  92 

53.  The  Electric  Telegraph        .        •.         ;•    - '.        .         .        ,  .    .  93 

54.  Intensity  of  Illumination.     The  Bunsen  Photometer  .         .  .  95 

55.  Law  of  Reflection  from  Plane  Mirrors         .         '*      :  .    "  ;  .  .  97 

56.  To  determine  the  Position  of  an  Image  in  a  Plane  Mirror  .  98 

57.  Spherical  Concave  Mirrors  .         .         .         .         .         .         .  .  100 

58.  The  Index  of  Refraction  of  Water       .         .        .         .-  -,     .  .  101 

59.  To  determine  the  Index  of  Refraction,  Air  to  Glass    .         .  .103 

60.  To  trace  the  Path  of  a  Ray  of  Light  from  Air  through  a  Tri- 

angular Glass  Prism  and  into  the  Air  again.     The  Angle  of 

Deviation       ' -.        .*-,.','    ,        .        .    :    :  .  104 

61.  To  determine  the  Focal  Length  of  a  Converging  Lens        .  .  105 

62.  Conjugate  Foci  of  a  Converging  Lens      '    .        .        .        .  .  105 

63.  The  Study  of  a  Photographic  Lens      .      „        .        ,.;       .  .  106 


PHYSICAL    LABORATORY    HANDBOOK 

SUGGESTIONS   FOR   EXPERIMENTING 

FOR  success  in  the  experimental  work  outlined  in  this  book 
it  is  necessary  that  the  following  general  directions  be  care- 
fully observed. 

Read  the  directions,  with  care,  before  beginning  to  make 
the  experiment. 

In  every  step  in  the  making  of  the  experiment  and  in  the 
keeping  of  the  notes,  be  accurate,  careful,  precise,  and  neat. 

Record  all  observations  at  the  time  they  are  made,  in  such 
a  way  that  the  record  will  be  permanent.  Care  should  be 
taken  to  record  the  exact  observations  made,  whether  these 
are  in  agreement  with  previous  ideas  or  not. 

Observations  should  be  repeated  under  the  same  conditions 
until  it  is  evident  that  correct  results  are  obtained.  The  final 
reading  should  be  the  average  of  a  set  of  readings. 

In  all  cases  in  which  a  scale  reading  is  taken  an  estimate 
must  be  made  of  the  tenths  of  a  division  beyond  the  last  unit 
reading. 

The  conditions  best  suited  to  secure  the  greatest  accuracy 
should  be  chosen. 

The  graphical  method  (as  illustrated  in  Fig.  11,  p.  22)  is 
frequently  the  best  waty  of  recording  the  results  of  an  experi- 
ment so  that  they  can  be  easily  interpreted. 

When  the  observations  have  been  made  and  the  results 
obtained,  a  detailed  report  should  be  written,  containing :  — 

1.  A  statement  of  the  object  of  the  experiment  and  the 
method  employed. 

7 


8  IN  TRODUCTORY 

2i    A  description  cf  the  apparatus  used. 

3.  A  proof  of  the  accuracy  'of  each  formula  when  first  used. 

4.  All  the  original  data  and  a  description  of  all  work  done 
in  obtaining  results. 

5.  The  curves  obtained  by  the  graphical  method  if  that  is 
applicable. 

6.  A  comparison  of  the  results  obtained  with  former  pub- 
lished results. 

Word  and  arrange  the  report  accurately  and  clearly. 

Guard  against  the  following  kinds  of  errors:  errors  of 
method;  inaccuracies  of  instruments;  accidental  errors  of 
observation ;  mistakes  in  calculation. 

However  well  the  experiments  may  have  been  made  and 
however  accurately  the  results  have  been  recorded,  the  stu- 
dent should  not  be  satisfied  unless  he  gets,  as  the  result  of  this 
work,  a  better  grasp  of  the  phenomena  and  their  interpreta- 
tion. 

HINTS   FOE   SIMPLE   LABORATORY  PROCESSES 

There  are  certain  simple  laboratory  processes  about  which 
every  worker  in  a  laboratory  should  know.  Successful  meth- 
ods of  working  will  come  to  those  only  who  are  willing  to 
work  for  them.  Personal  manipulation  alone  can  give  facility 
in  laboratory  working,  and  the  hints  that  are  given  here  are 
simply  directions  concerning  the  things  that  every  one  should 
know. 

To  clean  glass. — Wash  thoroughly  in  succession  with 
chromic  acid,  distilled  water,  and  alcohol.  In  some  stubborn 
cases  caustic  potash  may  be  used,  after  which  the  dish  must 
be  thoroughly  rinsed  with  distilled  water.  Caustic  soda  must 
not  be  used,  as  it  would  corrode  the  glass. 

To  dry  out  a  glass  flaslc.  —  Force  dry  air  into  it  through  a 
long  glass  tube  that  is  kept  hot  by  a  Bunsen  flame.  Use  a  foot 
pump  for  forcing  the  air  and  see  that  it  is  kept  free  from  dust. 


HINTS  EOR  SIMPLE  LABORATORY  PROCESSES 


To  clean  mercury.  —  Draw  out  one  end  of  a  glass  tube,  of 
5  or  6  min.  internal  diameter  and  a  meter  long,  into  a  narrow 
tube,  and  bend  it  into  a  double  U  as  shown  in  Fig.  1.  Fix  it 
in  a  vertical  position  and  pour  into  it  a  little  clean  mercury 
to  fill  the  lower  end.  Fill  the  tube 
nearly  to  the  top  with  a  10  %  solution 
of  nitric  acid.  Put  a  cork  stopper  in 
the  top  of  the  tube,  through  which 
pass  a  funnel  that  has  been  drawn 
out  to  a  capillary  end.  Pour  the  mer- 
cury to  be  cleaned  into  the  funnel.  It 
will  fall  through  the  acid  in  a  stream 
of  fine  drops  and  may  be  collected  in 
a  dish  placed  to  receive  it  at  the  end 
of  the  U  tube. 

To  cut  a  glass  tube.  —  (a)  If  the 
tube  is  small :  Cut  a  nick  in  the  tube 
at  the  proper  place  with  a  sharp  trian- 
gular file.  Grasp  the  tube  with  both 
hands  with  the  nick  outward.  Place  a 
thumb  on  each  side  of  the  nick  on  the 
opposite  side  of  the  tube.  Bend  the 
tube  back  across  the  tips  of  the  thumbs 
and  pull  it  apart  at  the  same  time,  and 
the  tube  will  come  in  two  with  even 
ends.  (6)  If  the  tube  is  large:  File 
a  deep  nick  in  the  tube.  Wind  two 
long  strips  of  filter  paper  around  the 
tube,  one  on  each  side  of  the  nick,  and 

tie  them  on  with  thread.  The  edges  of  the  paper  facing  the 
nick  should  be  even  and  parallel,  and  not  more  than  an 
eighth  of  an  inch  apart.  Dampen  the  paper  with  a  few 
drops  of  water.  Start  a  crack  at  the  nick  with  the  fine  point 
of  a  blowpipe  flame  and  lead  it  around  the  tube.  If  care- 
fully done,  this  method  will  give  even  ends. 


FIG.  1. 


CLEANING  MER- 
CURY. 


10  INTRODUCTORY 

To  bore  a  hole  in  a  glass  plate.  —  (a)  When  the  hole  is 
small :  Use  for  a  drill  the  end  of  a  small  rat-tail  or  triangular 
file,  moistening  the  glass  with  turpentine  in  which  camphor 
has  been  dissolved.  Work  the  drill  by  hand,  and  if  it  becomes 
dull,  break  off  a  piece  of  the  end  with  forceps. 

(b)  When  the  hole  is  large :  Cut  two  pieces  of  board  each 
a  little  larger  than  the  glass  plate  which  is  to  have  the  hole 
cut-in  it.  Bore  a  hole  in  the  upper  board  and  place  the  glass 
plate  between  them  so  that  this  hole  shall  come  exactly  over 
the  place  in  the  glass  plate  where  the  hole  is  to  be  cut.  Clamp 
the  boards  together  with  wood  screws.  Use  a  piece  of  gas 
pipe  for  a  drill  and  feed  it  with  emery  or  carborundum,  and 
keep  it  moist  with  the  camphor  solution.  Eotate  this  drill  by 
rubbing  between  the  hands,  pressing  lightly  upon  the  drill. 
It  is  a  good  plan  to  reverse  the  plate  and  drill  from  the  other 
side  when  the  hole  is  nearly  through. 

To  draw  a  capillary  tube.  —  Choose  a  fish-tail  burner  that 
gives  an  even  flame  two  and  a  half  inches  wide.  Heat  the 
middle  of  a  piece  of  small  tubing  20  cm.  long  in  this  flame. 
Hold  the  tube  in  the  luminous  part  of  this  flame  until  it 
softens  and  bends  easily.  When  it  is  uniformly  heated,  take 
it  out  of  the  flame  and  draw  it  to  the  required  dimensions  with 
a  slow,  steady  pull. 

To  bend  a  glass  tube.  —  Heat  the  tube,  at  the  part  where  the 
bend  is  to  be,  in  the  luminous  flame  of  a  fish-tail  burner.  Ro- 
tate the  tube  in  the  fingers  until  it  is  equally  heated,  and  until 
it  will  bend  freely.  Remove  it  from  the  flame  and  bend  it  to 
the  desired  form  by  a  steady  pressure.  Do  not  pull  the  tube 
much  or  it  will  be  reduced  at  the  bend.  If  the  Bunsen  burner 
is  used,  there  is  danger  of  heating  the  tube  unevenly,  so  that 
the  bend  will  not  be  symmetrical. 

To  close  the  end  of  a  glass  tube.  —  Break  the  tube  squarely 
off  at  the  point  selected.  Hold  the  end  vertically  in  the  hot- 
test part  of  the  Bunsen  flame  and  turn  it  slowly  until  the  end 


HINTS   FOR    SIMPLE   LABORATORY    PROCESSES  11 

is  melted  and  closed.  The  molten  glass  will  run  together  into 
the  form  of  a  sphere,  just  as  any  liquid  will  as  the  result  of 
capillarity,  and  the  end  will  be  evenly  rounded  if  the  heating 
is  right. 

To  use  sealing  wax.  —  In  order  to  make  a  satisfactory  seal- 
ing-wax joint  it  is  necessary  that  the  articles  should  first  be 
heated  and  have  a  thin  coating  of  the  wax  applied.  Reheat 
both  articles  until  the  wax  is  melted  and  then  press  them 
together  until  they  are  cooled  and  the  wax  is  hardened. 

Universal  wax.  —  A  form  of  soft  wax  that  is  very  useful 
for  making  temporary  joints  and  connections  is  made  as  fol- 
lows :  Thoroughly  work  together  4  parts,  by  weight,  of  bees- 
wax and  1  part  of  Venetian  turpentine.  Color  it  by  mixing 
with  this  a  sufficient  quantity  of  red  vermilion.  With  this 
wax  a  joint  can  be  made  by  pressure  alone,  without  heating. 

To  glue  wood  togetlxer.  —  Shape  the  parts  to  be  glued 
together  so  that  they  fit  each  other  accurately.  Heat  each 
part  and  coat  it  with  a  thin  coating  of  glue.  Reheat  and 
press  the  parts  firmly  together  and  leave  them  under  pressure 
until  the  glue  has  hardened. 

To  solder  a  wire  joint. — First,  clean  each  wire  thoroughly. 
This  is  best  done  with  chloride  of  zinc,  which  can  be  made  by 
dropping  small  pieces  of  zinc  into  dilute  hydrochloric  acid  — 
enough  zinc  should  be  put  in  to  make  a  saturated  solution. 
"  Tin  "  each  clean  wire  separately  with  the  solder.  Ordinary 
tinman's  solder  will  answer  for  most  laboratory  purposes. 
Wind  the  tinned  ends  of  the  wires  together  and  heat  in  the 
flame  of  an  alcohol  lamp,  adding  more  solder  if  needed. 

To  solder  with  the  soldering  iron.  —  In  order  that  a  solder- 
ing iron,  or  copper,  shall  work  properly  it  must  be  thoroughly 
clean.  Chloride  of  zinc,  resin,  or  borax  may  be  used  for  the 
purpose  of  a  flux.  Turn  up  the  edges  of  a  small  piece  of  tin 
plate  and  in  it  put  a  little  flux  (a  few  drops  of  zinc  chloride, 
for  example)  and  a  small  bit  of  solder.  Heat  the  iron  until 


12  INTRODUCTORY 

it  will  melt  the  solder,  and  then  work  it  back  and  forth  across 
the  tin,  and  the  solder  will  flow  over  the  entire  surface  of  the 
iron.  This  is  called  "tinning"  the  iron.  If  the  surfaces  of 
the  metals  to  be  soldered  are  clean,  and  the  iron  hot  enough, 
—  about  200°  C.  for  ordinary  solder, — a  good  job  can  readily 
be  obtained  with  a  little  care. 

In  order  that  the  soldered  joint  may  be  a  permanent  one  it 
must  be  washed  thoroughly  to  remove  all  traces  of  the  zinc 
chloride.  Eesin  or  some  form  of  commercial  flux  is  better 
than  zinc  chloride  for  use  in  soldering  joints  that  are  to  be 
used  for  electrical  work. 


EXPERIMENT   1 


Measurements  of  Lengths,   Areas,    and  Volumes.     Ele- 
ments of  Physics,  pp.  13-15 

Apparatus.  —  A  rectangular  block  of  wood,  having  three 
different  dimensions;  two  scales,  one  having  centimeter  and 
millimeter  divisions  and  the  other  inches  and  parts  of  the 
inch ;  an  overflow  tank  and  catch  basin ;  a  graduate ;  paraffin ; 
gas  flame  or  lamp. 

Method.  —  Rest  the  block  firmly  upon  a  table  and  place  the 
scale  upon  it  in  such  a  way  that  the  edge  upon  which  the 
divisions  are  marked  shall  come  next  to  the  block.  Move 
the  scale  until  some  one  divi- 
sion is  exactly  opposite  one 
edge  of  the  block  (Fig.  2),  and 
read  the  other  edge  by  taking 
the  whole  number  of  divisions 
plus  the  tenths  of  a  division 
between  the  last  mark  on  the 
scale  coming  upon  the  block  and 
the  edge  of  the  block.  The 

difference  between  the  readings  of  the  two  ends  is  the  length 
of  the  block.  The  estimation  of  tenths  of  a  division  is  very 
important  in  measurement,  and  the  accuracy  with  which  it  is 
done  makes  a  large  part  of  the  difference  between  a  good  meas- 
urement and  a  poor  one.  Suppose  that  the  width  of  the  block 
is  greater  than  15  and  less  than  16  mm.  as  shown  in  Fig.  2. 
The  value  of  the  reading  will  depend  upon  the  judgment  of 
the  reader.  The  reading  of  Fig.  2  should  be  15.4,  not  15.3  or 
15.5. 

13 


FIG.  2. 


MEASUREMENTS 


If  the  block  is  not  perfectly  rectangular,  take  several  meas- 
urements of  each  side  and  take  the  average  of  all  of  these  for 

the  dimension  of  that  side. 

Compute  the  areas  of  all  of  the  faces  of  the  block  in  both 

square    centimeters    and   square   inches.     Area  —  Length  x 

Breadth. 

Compute  the  volume  of  the  block  in  both  cubic  centimeters 

and  cubic  inches.     In  a  rectangular  block,  Volume  =  Area  of 

end  X  Length. 

Melt  a  piece  of  paraffin  in  a  small  dish  and  spread  it  over 

the  sides  of  the  block.     Heat  the  block  over  the  flame  until 

the  paraffin  is  absorbed  by 
the  wood.  This  fills  the 
pores  of  the  wood,  and  pre- 
vents the  absorption  of 
water.  Fill  the  overflow 
tank  (Fig.  3)  with  water 
and  let  the  overflow  run 
into  the  catch  basin. 
Empty  the  basin,  replace 
it  under  the  spout  of  the 

tank,  and  then  place  the  block  in  the  tank.     Submerge  the 

block  by  pushing  it  below  the  surface  of  the  water  with  the 

points  of  three  or   four  pins  that  have 

been  pushed  through  a  thin  piece  of  wood 

or  a  heavy  card.     Find  the  volume  of  the 

displaced    water    by   measuring   it   in   a 

graduate.      In    reading    the    surface    of 

water  in  a  graduate  read  the  bottom  of 

the  curved  surface,  as  shown  in  Fig.  4, 

and  estimate  to  the  tenths  of  the  smallest 

-,.    .  .  FIG.  4. 

division. 

Conclusion.  —  How  does  the  computed  volume  compare  with 
the  volume  of  water  forced  out  of  the  tank  by  the  submerged 
block  in  the  displacement  method? 


FIG.  3. 


THE    SURFACE   AND   VOLUME    OF  A   CYLINDER          15 

Suggested  Experiments.  —  Find  by  measurement  the  dimen- 
sions and  compute  the  areas  of  (a)  a  right  triangle ;  (&)  an 
isosceles  triangle ;  (c)  a  page  of  this  book,  each  margin,  and 
the  part  covered  by  the  printing. 

Find  by  the  displacement  method  the  volume  of  an  irregular 
solid. 

Find  by  the  same  method  the  weight  of  water  displaced 
by  a  floating  body,  and  compare  it  with  the  weight  of  the  body. 

EXPERIMENT   2 

To  find  the  Surface  and  Volume  of  a  Cylinder  and  of  a 
Sphere.     El.  of  Phys.,  p.  15 

Apparatus.  —  A  cylinder  of  hard  wood  (a  section  of  curtain 
pole  will  answer) ;  a  steel  ball  such  as  is  used  in  ball  bear- 
ings (a  large  one  is  preferable)  ;  a  vernier  caliper x  (if  there  is 
one  in  the  laboratory) ;  two  rectangular  blocks  ;  scales. 


FIG.  5.    VERNIER  CALIPER. 


1  The  vernier  is  used  to  measure  accurately  fractional  parts  of  the  smallest 
divisions  of  the  scale.  The  simplest  form  is  one  in  which  nine  of  the 
smallest  divisions  —  mm.  for  example  — 
are  divided  into  ten  equal  parts  which  are 


I 


I  i   i   i  i   i   i 

placed  upon  a  movable  scale  that  slides    B         l— 

along  the  first.    The  article  to  be  meas-     P  FIG.  6. 

ured  is  placed  between  the  fixed  end  P 

and  the  end  of  the  sliding  vernier  V  (Fig.  6),  and  the  reading  is  taken  from 

both  the  scale  and  the  vernier.     If  the  scale  in  Fig.  6  represents  millimeters, 

the  reading  is  5.3  mm.    Since  the  length  of  each  division  of  the  vernier  is  0.9 


7 


16  MEASUREMENTS 

Method.  —  (a)  To  find  the  area  of  a  cylindrical  sur- 
face. 

Measure  the  length  of  the  cylinder  in  centimeters  and  inches. 
Measure  its  circumference  as  follows :  Lay  the  cylinder  on  a 
sheet  of  paper  so  that  a  radial  mark  on  the  end  is  exactly  over 

a  point  P,  marked  on 
the  paper  (Fig.  7).  Roll 
the  cylinder  over  once, 
being  careful  that  it  does 

FIG  7  not  slip,  and  make  a  mark 

P'  on  the  paper  where 
the  radial  mark  on  the  end  of  the  cylinder  touches  it  the 
second  time.  Measure  the  distance  between  the  two  points. 
This  gives  the  circumference  of  the  cylinder.  Compute  the 
area  from  the  formula,  Area  =  Length  x  Circumference. 

(b)  To  find  the  volume  of  a  cylinder. 

In  order  to  do  this  it  is  necessary  to  measure  the  diameter. 
This  can  be  done  accurately  by  using  a  vernier  caliper.  If 
this  is  not  at  hand,  place  the 
cylinder  between  two  rec- 
tangular blocks  and  measure 
the  distance  between  the 
blocks  by  placing  a  scale 
across  the  top  as  shown  in 
Fig.  8.  Do  this  for  different  FIG  8. 

diameters,  and  take  the  aver- 
age as  the  diameter  of  the  cylinder.     Substitute  the  proper 
numbers   in    the    formula  for    the   volume   of  the   cylinder, 

of  a  millimeter,  the  zero  end  of  the  vernier  is  0.3  mm.  from  the  5-mm.  mark 
on  the  scale  whenever  mark  3  of  the  vernier  is  opposite  the  8-mm.  mark  on 
the  scale ;  that  is,  the  mark  3  mm.  farther  on.  If  the  vernier  is  placed  so  that 
mark  6  of  the  vernier  is  opposite  any  mark  in  the  scale,  then  0.6  mm.  must 
be  added  to  the  millimeters  read  on  the  scale  up  to  the  zero  end  of  the  vernier. 
Figure  5  shows  the  method  of  using  the  vernier  caliper  in  measuring  the 
diameter  of  a  cylinder,  an  inside  diameter,  or  a  depth. 


THE    DIAMETER  OF   A    WIRE  17 

Volume  =  Length  x  Area  of  end,  and  find  the  volume  in  cubic 
inches  and  cubic  centimeters. 

NOTE.  —  Area  of  circle  =  ?rr2,  in  which  r  is  the  radius  of  the  circle  and 
TT  represents  the  ratio  of  the  circumference  of  a  circle  to  its  diameter,  or 
nearly  3.1416. 

(c)  To  find  the  surface  of  a  sphere. 

Measure  the  diameter  of  the  steel  ball  in  the  same  way  in  which 
you  measured  the  diameter  of  the  cylinder  in  (6).  Substitute 
the  proper  values  in  the  formula,  Surface  of  Sphere  =  4  -n-r2. 

(d)  To  find  the  volume  of  a  sphere. 

Substitute  proper  values  in  the  formula,  Volume  of  sphere 

->«*. 

Suggestion.  —  From  your  measurements  of  the  circumference 
of  the  cylinder  in  (a)  and  of  the  diameter  in  (6)  compute  the 
value  of  TT.  The  more  accurate  your  measurements,  the  nearer 
your  result  will  approximate  the  true  value. 

EXPERIMENT  3 
To  find  the  Diameter  of  a  Wire.     El.  of  Phys.,  p.  15 

Apparatus.  —  A  micrometer  caliper  *  and  several  •  sizes  of 
insulated  wire. 

1  The  micrometer  caliper,  or  sheet  metal  gauge,  is  an  instrument  for  the 
accurate  measurement  of  distances.  It  consists  of  a  U-shaped  piece  of  brass 
or  steel  A,  through  one  arm  of  which  is  threaded  a  screw  B,  which  is  turned  by 
the  milled  head  C,  or  by  the  ratchet  head  R,  and  which  may  be  locked  in  any 
given  position  by  means  of  the  collar  F.  On  the  inner  metal  sleeve  D  there  is 


FIG.  9.  MICROMETER  CALIPER. 
PHYS.  LAB.  BOOK 2 


18  MEASUREMENTS 

Method.  —  To  measure  the  diameter  of  an  insulated  wire  hold 
the  wire  between  jfif  and  the  end  of  B  (Fig.  9),  and  turn  R  up 
until  the  ratchet  clicks,  when  the  wire  will  be  lightly  held 


FIG.  9.    MICROMETER  CALJPER. 


between  the  jaws.  (If  your  caliper  has  no  ratchet  head  72, 
turn  (7;  but  C  should  be  held  loosely  between  the  thumb  and 
finger,  and  no  pressure  must  be  put  upon  it.)  The  diameter 
of  the  wire  will  be  the  sum  of  the  readings  on  the  scale  D  and 
the  circular  scale  E.  Take  five  different  readings  and  average 
the  result  for  the  diameter. 

Unwind  the  insulation  from  the  wire  and  take  readings 
of  the  diameter  of  the  uncovered  wire. 

Subtract  this  reading  from  the  first  to  get  the  double  thick- 
ness of  the  insulation. 

Look  in  a  wire  table  to  find  the  number  of  the  wire.  Note 
the  difference  between  your  measurement  and  the  diameter 
given  in  the  table.  Determine  the  per  cent *  of  difference. 

a  longitudinal  scale,  the  divisions  of  which  are  the  same  as  the  distance 
between  the  threads  of  the  screw.  On  the  end  of  the  outer  sleeve  E  there  is 
a  circular  scale,  divided  into  a  number  of  equal  parts.  When  the  milled  head 
C  is  turned  through  one  complete  turn,  the  end  of  the  screw  B  moves  toward 
or  away  from  K  a  distance  that  is  equal  to  the  pitch  of  the  screw,  one  milli- 
meter, for  example.  If  the  circular  scale  has  50  equal  parts  and  C  is  turned 
the  distance  of  one  of  these  parts,  the  end  of  B  moves  through  one  fiftieth  of 
a  millimeter.  When  the  end  of  B  touches  the  face  of  K,  both  scales  should 
read  zero.  If  they  do  not,  make  a  record  of  the  reading,  which  must  be  added 
to  or  subtracted  from  all  future  readings. 

1  The  per  cent  of  difference  may  be  obtained  by  dividing  the  difference  be- 
tween the  reading  and  the  diameter  given  in  the  table  by  the  latter. 


DENSITY 

Record  your  results  somewhat  as  follows:  — 


19 


NUMBER  OF  READIN:; 

riAMETER    WITH    INSULATION 

DIAMETER  WITHOUT  INSULATION 

1       ...... 

1.81  mm. 

1.02  min. 

2     

1.87 

1.01 

3     ...... 

1.85 

1.01 

4     

1.82 

1.02 

5     

1.78 

1  01 

Sum     
Average  .... 

9.13 
1.826 

5.07 
1.014 

Thickness  of  insulation  =  (1.826-  1.014)  H-  2  =  0.406  mm. 
1.014  mm.  =0.03992  in.,  or  39.92  mils.  (1  mil  =  0.001  inch.) 
The  number  nearest  to  this  in  the  table  (El.  of  Phys.,  p.  330) 
is  40.3,  and  this  number  is  for  wire  No.  18.  The  per  cent  of 
difference  is  (40.3  -  39.92)  -*-  40.3  =  0.009  =  0.9  %. 

Suggested  Experiments.  —  Use  the  micrometer  caliper  to 
determine  the  volume  of  small  cylinders  and  bicycle  balls. 
Measure  the  thickness  of  sheet  metal,  tin,  and  brass,  and  of 
the  paper  in  this  book. 

EXPERIMENT   4 

To  find  the  Mass  per  Unit  Volume  or  the  Density  of  a 
Body-     El.  of  Phys.,  p.  16 

(a)  When  the  body  is  of  regular  form. 

Apparatus.  —  The  rectangular  block  used  in  Experiment  1 ; 
a  balance  and  weights. 

Method. — Weigh  the  block  and  divide  its  mass  thus  found 
by  its  volume  already  found.  This  will  give  the  density  of 
the  block  or  its  mass  per  unit  volume. 

The  density  of  a  body  is  usually  given  in  grams  per  cubic 
centimeter.  It  may  be  given  in  pounds  per  cubic  foot.  Com- 
pute the  density  of  the  block  in  terms  of  these  units. 


20  MEASUREMENTS 

To  weigh  the  body,  first  examine  the  balance  to  see  if  the 
pointer  stands  at  zero  when  the  pans  are  empty.  If  it  does 
not,  turn  the  adjusting  screw  at  the  end  of  one  of  the  arms, 
until  the  pointer  reads  zero.  Find  by  trial  whether  the  pans 
are  interchangeable  or  not.  If  they  are  not,  mark  the  pans 
"right"  and  "left."  Place  the  body  upon  one  pan  and  a 
weight  that  you  think  is  heavier  upon  the1  other  pan.  If  the 
weight  selected  is  lighter  than  the  body,  replace  it  by  a  heavier 
one.  If  it  is  heavier  than  the  body,  remove  it  from  the  pan  and 
put  on  the  next  lighter  one.  Repeat  until  you  have  a  weight 
lighter  than  the  body.  Leave  this  weight  on  the  pan  and  add 
the  next  lighter  weight.  Continue  the  process  by  taking  every 
weight  in  order,  putting  back  into  the  weight  box  those  that 
are  too  heavy  and  leaving  the  others  on  the  pan  until  the  body 
is  balanced.  The  sum  of  the  weights  in  the  pan  is  the  weight 
of  the  body.  Always  use  the  pincers  provided  for  lifting  the 
weights,  and  place  them  upon  the  pans  carefully.  A  support 
placed  under  one  of  the  pans  will  reduce  the  swing  and  bring 
the  arms  to  balance  more  quickly. 

(b)  When  the  body  is  of  irregular  form. 

Apparatus.  — A  graduate  ;  balance  ;  weights  and  thread. 

Method. — Weigh  the  body  and  call  its  mass  m.  Pour  water 
into  the  graduate  and  take  a  reading.  Tie  a  thread  to  the 
body  and  suspend  it  in  the  water.  See  that  all  air  bubbles  are 
removed  from  it.  Take  a  second  reading.  Take  the  difference 
between  the  two  readings  as  the  volume  of  the  body  and  call 
it  v.  If  the  graduate  is  calibrated  in  cubic  centimeters,  the 
density  will  be  found  by  dividing  m  by  v. 

Conclusion.  —  How  do  you  know  that  the  difference  between 
the  two  graduate  readings  is  the  volume  of  the  body  ?  Why 
should  all  air  bubbles  be  removed  from  the  submerged  body? 

Suggested  Experiments.  —  Find  the  density  of  the  cylinder 
and  sphere  measured  in  Experiment  2,  after  using  paraffin  to 
protect  the  wood  as  in  Experiment  1. 


EXPERIMENT   5 


To  study  the   Relation  between  the   Force  acting  upon 
an  Elastic  Body  and   the  Distortion  produced  by  it. 

EL  of  Phys.,  p.  22 

(«)  Elasticity  of  traction. 

Apparatus.  —  A  spiral  spring  (if  no  other  spring  can  be 
found,  make  use  of  the  kind  of  spiral  spring  used  to  close 
screen  doors) ;  a  metric  scale  mounted 
vertically  ;  a  set  of  weights. 

Method. —  Fasten  the  spring  rigidly  to 
a  support,  as  shown  in  Fig.  10.  Straighten 
out  the  wire  of  the  spring  at  the  lower  end 
and  solder  a  horizontal  pointer  to  it. 
Suspend  a  weight  pan  from  the  spring. 
Place  the  metric  scale  in  such  a  position 
that  the  end  of  the  pointer  is  close  to  its 
face.  Take  a  reading  in  millimeters  and 
tenths  (estimated)  and  record  this  as  the 
zero  reading.  Put  a  5-g.  weight1  in  the 
pan  and  read  the  elongation.  Take  a  series 
of  readings,  adding  5  g.  at  a  time,  until 
there  is  a  maximum  load  of  50  g.  Take 
a  second  series  of  readings  as  you  remove  the  weights,  5  g.  at 
a  time,  until  the  load  is  zero. 

Conclusion.  —  Take  the  average  of  the  two  readings  as  the 
elongation  for  each  different  load.  Draw  a  curve,  represent- 

1  The  size  of  wire  used  for  the  spring  will  determine  the  value  of  the 
weights  to  be  used  as  loads.  If  the  wire  is  large,  the  weights  would  need  to 
be  greater. 

21 


FIG.  10. 


22 


THE  PROPERTIES  OF  MATTER 


ing  the  loads  by  vertical  distances  and  the  elongations  of  the 
spring  by  horizontal  distances. 

In   accordance  with   Hooke's   Law  the  curve    should    be  a 


oo  -  - 

Kf) 

/ 

-- 

/ 

45 

/ 

/ 

t 

/ 

/ 

/ 

OK    _  . 

/ 

t 

30 

LOAD,           ELONGATION, 
GRAMS.      CENTIMETERS. 

0               00.00 
5                2.50 
10                5.22 
15                8.15 
20              11.10 
25              13.93 
30              16.81 
35              19.60 
40               22.54 
45               25.21 
50               28.25 

/ 

/ 

j 

y 

/ 

90 

_/ 

{_ 

t 

/ 

7 

/ 

t 

~J 

m 

Jt 

/ 

j 

10 


15 


20  25  30 

ELONGATION 

FIG.  11. 


35 


40 


45 


50 


straight   line.      Figure   11    shows   the   tabular   results  of  an 
experiment  with  the  curve  obtained  from  them. 

Suggested  Experiment.  —  Calibrate  the  scale  of  a  Jolly  bal- 
ance. (Find  the  value  of  the  scale  divisions.)  The  Jolly 
balance  is  a  sensitive  form  of  spring  balance  shown  in  Fig.  12. 


ELASTICITY 


23 


There  are  two  scale  pans,  the  lower  of 
which  can  oe  submerged  in  water  to 
check  the  vibrations  of  the  spring.  The 
readings  are  taken  by  sighting  across  the 
top  of  a  bead  carried  by  the  spring  wire 
and  noting  the  position  of  its  image  in  a 
mirror  scale  carried  by  the  standard  of 
the  balance. 

Are  the  divisions  on  an  ordinary 
spring  scale  the  same  distance  apart? 
Tell  why  in  your  notes.  A  satisfactory 
method  of  reading  the  elongation  of 
the  spring  in  the  Jolly  balance  is 
to  use  a  reading  telescope.  This  con- 
sists of  a  telescope  which  can  be  moved 
up  and  down  a  vertical  stand,  always 
pointing  in  a  horizontal  direction.  By 
placing  the  vertical  scale  directly  behind 
the  spring,  the  position  of  some  definite 
mark  at  the  lower  end  of  the  spring  can 
be  read  accurately. 


FIG.  12.    JOLLY 
BALANCE. 


(b)   Elasticity  of  bending. 

Apparatus.  —  A  rectangular  wooden  rod,  15  mm.  by  5  mm.  and 
over  a  meter  long;  vertical  metric  scale;  scale  pan;  weights; 
and  a  support,  shown  in  Fig.  13. 

Method. — Fix  the  uprights  exactly  one  meter  apart  and 
bore  a  hole  through  the  brace  between  them.  Place  the  rod 
on  its  side  on  the  uprights.  Suspend  the  scale  pan  from  the 
middle  of  the  rod,  carrying  the  suspension  through  the  hole 
in  the  brace,  and  set  up  the  scale  behind  the  rod.  Take  the 
zero  reading  when  only  the  weight  holder  is  on  the  rod.  Take 
a  series  of  readings  with  loads  of  50  g.,  100  g.,  and  so  on  up 
to  500  g.,  increasing  the  load  50  g.  for  each  reading.  Take 
a  second  series  of  readings  as  you  decrease  the  load  50  g.  at  a 


24  THE   PROPERTIES   OF   MATTER 

time  until  zero  load  is  reached.  Repeat  the  experiment,  turn- 
ing the  rod  on  its  edge. 

Conclusions.  —  From  the  average  results  in  each  experiment 
make  a  table  showing  the  relation  of  load  to  amount  of  bending. 

Compare  the  results  of  the  two  experiments  and  state  in 
your  notes  the  best  way  to  place  the  floor  timbers  of  a  house. 


FIG.  13. 

Suggestions.  —  Substitute  a  steel  rod  for  the  wooden  rod  and 
repeat. 

Fasten  the  rod  at  one  end  and  suspend  a  scale  pan  from  the 
other  and  make  the  experiment. 

If  your  laboratory  has  the  apparatus  for  finding  the  elasticity 
of  torsion,  make  that  experiment. 

EXPERIMENT  6 

To  find  the  Breaking  Strength  of  a  Wire.     El.  of  Phys., 

p.  25 

Apparatus.  —  A  wire  tester  (Fig.  14)  ;  spring  brass  wire  Nos. 
24  and  27  ;  micrometer  caliper. 

Method.  —  Thread  one  end  of  wire  No.  27  through  the  hole 
in  the  crank  shaft  of  the  tester  and  fasten  it.  Fix  the  proper 
length  of  wire  to  the  scale.  Turn  the  handle  of  the  tester, 


BREAKING   STRENGTH   OF  A   WIRE 


25 


being  careful  that  the  pawl  moves  freely  over  the  toothed 
wheel.  Push  down  steadily  upon  the  wedge  in  order  to  prevent 
the  flying  back  of  the  balance  when  the  wire  breaks.  Turn  the 
crank  handle  steadily  and  slowly  to  avoid  sudden  changes  in 
pressure. 

Make  three  tests  for  this  size  of  wire  and  take  the  average 
for  the  breaking  weight.  Measure  the  diameter  of  the  wire. 

Make  the  same  tests  with  wire  No.  24. 


FIG.  14.    WIRE  TESTER. 


Conclusions.  —  Compute  the  tensile  strength  of  each  wire  in 
kilograms  per  square  centimeter  of  cross  section. 

Is  the  breaking  weight  directly  proportional  to  the  area  of 
cross  section  ? 

Suggested  Experiments.  —  Test  the  breaking  weight  of  iron 
wire  and  compare  it  with  that  of  brass  of  the  same  cross  sec- 
tion. Do  the  same  for  copper  wire  and  different  kinds  of 
thread. 

Can  you  determine  how  much  a  copper  wire  will  stretch  be- 
fore it  will  break  ? 


EXPERIMENT  7 


To  determine  the  Conditions  of  Equilibrium  between 
Three  Concurrent  Forces  acting  in  the  Same  Plane. 
The  Parallelogram  of  Forces.  El.  of  Phys.,  p.  44 

Apparatus.  —  A  board  about  two  feet  square  with  a  row  of 
wire  nails  near  each  of  two  adjoining  sides,  and  a  row  of  holes 
near  each  of  the  other  sides ;  three  balances ;  cross-section 
paper ;  thumb  tacks. 

Method.  —  Fasten  the  ring  of  a  balance  to  a  nail  in  each  row, 
and  pull  the  third  in  any  desired  direction.  Fasten  it  in 

that  position  by  means  of  a 
wooden  or  metal  pin  pushed 
into  a  hole.  Pin  the  cross- 
section  paper  to  the  board  so 
that  the  intersection  of  two 
heavy  cross  lines  comes  di- 
rectly beneath  the  common 
point  to  which  the  balances 
are  fastened.  Make  a  pencil 
point  on  the  paper  directly 
under  the  connecting  point  of 
the  three  cords  joining  the 
balances.  Take  a  reading  of 
the  three  balances  and  mark 
the  points  to  which  they  are  attached.  Remove  the  balances 
and  draw  lines  from  the  common  point  to  the  points  of  support. 
Lay  off  on  these  lines  distances  that  are  proportional  to  the 
readings  taken.  Complete  the  parallelogram  on  any  two  lines 
as  sides  and  draw  the  resulting  diagonal. 

26 


FIG.  15. 


PARALLELOGRAM   OF   FORCES 


Conclusion.  —  How  does  the  resultant  compare  in  length  and 
direction  with  the  third  force  ? 

-  Any  force  that  is  opposite  in  direction  and  equal  in 
intensity  to  the  resultant  of  two  forces  is  called  an  equili- 
brant. 

Suggestion.  —  The  use  of  cross-section  paper  makes  it  con- 
venient to  make  a  study  of  the  relative  values  of  the  rectangular 
components  of  three  forces  that  are  in  equilibrium  in  one  plane. 
This  may  be  done  as  follows :  — 

Through  Ay  the  point  of  application  of  the  three  forces,  draw 
two  heavy  lines  at  a  right  angle  to  each  other  along  the  section 


FIG.  16. 


28  THE  MECHANICS  OF  SOLIDS 

lines.     Call  these  the  axes  X  and  Y.     By  projecting  the  forces 
upon  these  axes  it  will  be  seen :  — 

(a)  That  the  projection  of  R  on  the  axis  of  X  is  equal  to 
the  sum  of  the  projections  of  P  and    Q   on   that  axis ;    i.e. 
AM=AB  +  AC. 

(b)  That  the  projection  of  E  on  Fis  equal  to  the  difference 
of  the  projections  of  P  and  Q  on  that  axis  ;  i.e.  AL  =  AK—  AH. 

(c)  That  the  projections  of  E  on  X  and  Fare  equal  to  the 
projections  of  R  on  the  same  axis  but  in  the  opposite  direction. 

(d)  That,  in  general,  when  three  concurrent  forces  acting  in 
one  plane  are  in  equilibrium,  the  projection  of  any  one  of  these 
forces  upon  two  rectangular  axes  is  equal  to  the  algebraic  sum 
of  the  projections  of  the  two  other  forces  upon  these  axes. 

If  the  projections  of  the  forces  are  determined  by  means  of 
the  cross-section  lines,  this  equality  can  be  shown  by  counting 
the  small  squares  on  the  axes. 

The  pupil  should  make  several  determinations  of  equilibrium 
for  different  readings  and  positions  of  the  balances  and  then 
apply  the  data  found  to  the  determination  of  the  rectangular 
coordinates. 

EXPERIMENT   8 

Equilibrium   of  Three  Parallel   Forces   in  the  Same 
Plane.    El.  of  Phys.,  p.  47 

Apparatus. — A  light,  stiff  wooden  rod  more  than  a  meter 
long;  a  meter  stick;  two  spring  balances ;  a  two-pound  weight, 
or  a  kilogram  weight.1 

Method.  —  Mark  off  on  the  rod  the  length  of  a  meter  and 
divide  it  into  decimeters  by  cross  lines  drawn  with  a  sharp- 
pointed  pencil.  Suspend  the  balances  A  and  B  from  the  nails 
G  and  D  exactly  one  meter  apart,  and  suspend  the  rod  from 

1  The  weight  may  be  greater,  if  desired,  provided  it  is  somewhat  less  than 
the  total  amount  that  each  balance  can  measure,  and  can  be  supported  by  the 
wooden  rod  without  much  bending. 


PARALLEL   FORCES  29 

the  balances  by  means  of  loops  of  cord  at  the  points  E  and 
F,  one  meter  apart.  Take  a  reading  of  each  balance.  These 
readings  are  due  to  the  weight  of  the  rod  and  are  called  the 
"zero  readings."  They  do  not  enter  into  the  final  results, 
C  D 


i         i        i        i        i      -r 


E 


FIG.  17. 

which  must  be  corrected  by  subtracting  from  each  reading  the 
zero  reading  of  the  balance  on  which  it  is  taken. 

Suspend  the  weight  W  by  a  loop  on  the  rod  at  the  first 
mark,  directly  under  balance  A.  Head  both  balances.  Move 
W  one  decimeter  to  the  right  and  again  read  both  balances. 
Move  W  to  the  second  decimeter  division  and  read  again. 
Eepeat  the  readings,  moving  W  one  decimeter  each  time  until 
it  hangs  directly  under  the  balance  B. 

Tabulate  the  readings  as  follows  :  — 

Distance  from  E    \    Reading  of  A    \    Reading  of  B    \    Distance  from  F 

Conclusions. —The  meaning  of  the  results  given  in  the  table 
can  be  best  seen  by  making  a  curve  which  shall  have  the  dis- 
tance EF  laid  off  along  the  horizontal  axis  and  the  pull  on 
each  balance  for  the  different  positions  of  W  on  the  vertical 
axis.  The  curves  for  both  balances  can  be  made  on  the  same 
sheet  of  paper,  and  a  comparison  of  the  two  will  aid  in  under- 
standing how  the  pull  of  W  is  distributed  to  A  and  B  accord- 
ing to  its  position  on  EF. 


30 


THE   MKCUANiCS   OF   SOLIDS 


Suggestions.  —  State  the  relation  between  the  sum  of  the 
forces  acting  in  one  direction  and  the  force  acting  in  the  op- 
posite direction.  State  the  relation  between  the  outside  forces 
and  their  respective  distances  from  the  force  between  them. 


EXPERIMENT   9 
Laws  of  the  Pendulum.    El.  of  Phys.,  p.  81 

Apparatus.  —  Some  form  of   support 1 ;    linen  thread  ;  balls 

of  steel,  lead,  and  wood ;  a  watch  with  a  second  hand,  or  a  stop 

watch. 
Method.  —  (a)  To  find  the  time  of  vibration,  or  period, 

of  a  pendulum. 

Fasten  the  thread  to  a  steel  ball  with  wax  and  adjust  the 

thread  to  any  convenient  length,  two  or  three  feet,  for  example, 
Set  up  a  board  behind  the  pendulum 
and  draw  a  vertical  chalk  mark  exactly 
behind  the  thread  when  it  is  at  rest 
(Fig.  18).  Draw  the  bob  aside  about  10 
cm.  and  let  it  swing.  Let  one  pupil 
count  the  vibrations  aloud  just  as  the 
thread  passes  the  chalk  line.  Count  the 
passages  as  follows :  3-2-1-GO-1-2-3, 
and  so  on.  This  gives  the  second 
pupil  who  is  taking  the  time  a  fair 
warning  of  the  "  GO  "  when  the  time  is 
to  begin.  Take  the  time  of  50  or  100 
vibrations,  the  observer  giving  a  sharp 

"GO"  at  the  100th   swing,  and   the   timekeeper  reading   the 

minutes  and  seconds  and  estimating  the  tenths  of  a  second. 

Make  three  experiments  and  take  the  average  of  the  three 

times  as  the  true  time. 

1  Bore  holes  in  a  narrow  board  and  fit  corks  to  the  holes.  Cut  slits  half- 
way through  the  corks  with  a  sharp  knife  and  slip  the  suspending  threads 
into  the  slits.  The  friction  will  hold  the  thread  in  place. 


FIG.  18. 


LAWS   OF   THE    PENDULUM  31 

Divide  the  time  by  the  number  of  vibrations  to  find  the 
period. 

(6)  To  find  whether  the  amplitude  affects  the  period 
or  not. 

Make  the  same  experiment  with  the  same  pendulum  swung 
at  different  amplitudes.  Make  one  experiment  with  a  very 
large  amplitude. 

(c)  To  find  out  whether  the  mass  of  the  bob  changes  the 
period  or  not. 

Suspend  a  second  pendulum  of  the  same  length  as  the  first, 
but  made  with  a  larger  ball.  To  find  the  length  add  the  radius 
of  the  ball  to  the  distance  between  the  bottom  of  the  cork  and 
the  top  of  the  ball.  Compare  the  times  of  vibration  of  the 
two. 

(d)  To  find   the  relation   between  the   lengths   of  two 
pendulums  when  their  periods  are  as  1  ;  2. 

Find  the  period  of  a  pendulum  having  as  great  a  length  as 
the  apparatus  will  allow.  Measure  from  the  center  of  the  ball 
to  the  point  of  support.  Suspend  a  second  pendulum  like  the 
first  and  shorten  it  until  it  vibrates  twice  while  the  first 
pendulum  is  vibrating  once.  Measure  its  length  and  compare 
the  two. 

Conclusions.  —  Does  a  small  change  in  amplitude  change  the 
period  ?  Does  a  large  change  in  amplitude  change  the  period  ? 
Explain.  Does  a  change  in  the  mass  of  the  bob  change  the 
period  ?  Do  the  results  of  (d)  verify  the  law  ? 

Suggestions.  —  Put  your  values  of  t  and  I  in  the  formula 

t  =  2  «••%/-  and  find  the  value  of  g.     Make  a  graph  —  a  curve  — 

that  shall  show  the  relation  between  the  lengths  and  periods 
of  simple  pendulums. 


32 


THE   MECHANICS   OF   SOLIDS 


EXPERIMENT    10 
The  Moments  of  Forces.    El.  of  Phys.,  p.  88 

Apparatus.  —  Three  spring  balances;  two  weights;  a  very 
stiff  wooden  rod  more  than  a  meter  long ;  a  meter  stick  ;  a 
level. 

Method.  —  Suspend  two  of  the  balances  by  cords  from  hooks 
one  meter  apart,  and  suspend  the  third  between  them,  as  in 
Fig.  19.  Take  the  reading  of  each  balance  and  record  it  as 
the  zero  reading. 

Weigh  the  rod.  Suspend  the  rod  from  the  three  balances 
by  cords  as  shown,  so  that  it  shall  be  horizontal.  Suspend 


1) 


FIG.  1<). 

the  two  weights  from  the  rod  at  convenient  points,  as  at  G 
and  H,  and  so  adjust  the  cords  supporting  the  balances  that 
the  rod  shall  be  horizontal.  Assume  that  the  forces  tend  to 
turn  the  rod  about  the  point  E,  or  that  E  is  the  center  of 
moments.  Consider  that  the  weight  of  the  rod  itself  is 
applied  at  its  middle  point.  Compute  the  moment  of  each 
force,  and  determine  whether  it  tends  to  turn  the  rod  around 
the  point  E  in  a  clockwise  or  a  counter-clockwise  direction. 
Make  a  computation  of  the  moments  of  the  forces  and 
whether  they  tend  to  produce  clockwise  or  counter-clockwise 


THE   MOMENTS   OF   FORCES 


33 


rotation,  taking,  in  turn,  each  of  the  points  D,  G,  //,  and  F  as 
the  center  of  moments.  Do  the  same,  taking  any  other  point 
7T  as  the  center  of  moments. 

Conclusion.  —  How  does  the  sum  of  the  readings  of  the  bal- 
ances compare  with  the  sum  of  the  suspended  weights  ?  •  How 
does  the  sum  of  the  moments  that  tend  to  produce  clockwise 
rotation  compare  with  the  sum  of  those  that  tend  to  produce 
counter-clockwise  rotation  ?  Does  it  make  any  difference  in 
this  comparison  what  point  you  take  as  the  center  of  moments  ? 

Suggestions.  — Explain  in  your  notes  how  this  experiment 
illustrates  the  different  classes  of  levers.  Make  such  changes 
in  the  apparatus  as  are  necessary  for  the  purpose,  and  test  the 
law  for  each. 

EXPERIMENT   11 

To  determine  whether  the  Point  of  Application  of  the 
Weight  of  a  Lever  is  in  the  Same  Position  as  its 
Center  of  Gravity-  El.  of  Phys.,  p.  88 

Apparatus.  —  A  straight  wooden  bar  about  two  feet  long  j  a 
wooden  block;  a  weight ;  triangular  fulcrum ;  try -square. 

Method.  —  Make    a    saw 

cut  A  a  quarter  of  an  inch       *  ^      \  B 

deep  near  one  end  of  the 
bar  and  screw  the  block  B 
to  the  other  end.  Place 
the  triangular  fulcrum  F 
on  the  table  and  balance  the 
bar  upon  it.  This  will  be 
more  easily  done  if  the  ful- 
crum edge  of  the  block  is 
planed  off  until  it  is  about 
one  eighth  of  an  inch  wide.  When  the  bar  is  exactly  balanced, 
mark  a  point,  (7,  directly  above  the  middle  of  the  fulcrum  edge 
and  draw  a  fine  pencil  mark  across  the  bar,  using  the  try- 

PHYS.   LAB.   BOOK 3 


FIG.  20. 


34  THE  MECHANICS   OF   SOLIDS 

square  to  get  it  perpendicular  to  the  side.  Bring  the  fulcrum 
near  one  side  of  the  table.  Suspend  the  weight  from  the  bar 
by  means  of  a  cord  in  the  saw  cut  near  the  end,  and  balance 
the  bar  and  weight  as  shown  (Fig.  20). 

Conclusion.  —  From  the  data  which  you  have  found  determine 
the  moment  of  the  weight,  taking  the  position  of  the  fulcrum 
as  the  center  of  moments. 

Determine  also  how  far  from  the  fulcrum  the  weight  of  the 
bar  and  block  must  be  applied  so  that  its  moment  may  be  equal 
to  the  moment  of  the  suspended  weight. 

How  does  this  distance  compare  with  the  distance  of  the 
center  of  gravity  of  the  ,bar  and  block  from  the  fulcrum  ? 

Where  may  you  consider  the  weight  of  a  body  to  be  ap- 
plied ? 

Suggestions.  —  From  the  data  that  you  have  found  prove  that 
this  is  a  lever  of  the  first  class. 

Compute  the  pressure  applied  at  the  fulcrum. 


EXPERIMENT   12 
The  Inclined  Plane.     El.  of  Phys.,  p.  96 

Apparatus.  —  Some  form  of  inclined  plane ;  small  car ; 
weights. 

Method.  —  Raise  one  end  of  the  plane  until  it  makes  an 
angle  of  about  30°  with  the  base.  Place  the  car  upon  the 
plane,  attach  the  scale  pan  as  shown  in  Fig.  21,  and  place 
weights  in  the  scale  pan  until  the  car  will  move  slowly  up  the 
plane  when  the  plane  is  lightly  tapped  with  the  finger.  Re- 
move weights  until  the  car  will  move  slowly  down  the  plane 
when  it  is  tapped.  Weigh  the  scale  pan  and  add  its  weight 
to  that  of  its  contents  in  each  case.  Take  half  the  sum  of  the 
two  weights  as  the  true  weight  when  the  car  is  at  rest.  Take 
the  weight  of  the  car  and  its  contents  as  the  weight  of  the 
load.  Repeat  at  a  larger  angle. 


FRICTION 


35 


Conclusions.  —  Substitute  your  results  in  formula  35,  namely, 
P:  W=H:L,  and  find  the  percentage  of  difference  between 
your  value  of  P  and  that  required  by  formula  35. 

Calculate  the  work  necessary  to  move  the  car  along  the 
plane  from  one  point  to  another.  Calculate  also  the  work 


FIG.  21. 


necessary  to  lift  the  car  vertically  through  the  distance  meas- 
uring the  difference  in  level  of  the  two  points.  How  do  these 
two  amounts  of  work  compare  ? 


EXPERIMENT   13 

Friction.     El.  of  Phys.,  p.  100 

Apparatus.  —  A  block  of  wood  with  sides  of  equal  smooth- 
ness ;  a  smooth  hardwood  board ;  spring  balance ;  weights. 

Method. — Place  the  board  upon  a,  table  and,  if  necessary, 
block  it  up  until  it  is  level.  Place  the  block  resting  on  its 
wide  face  upon  the  board  and  put  a  1-kg.  weight  upon  it. 
Attach  the  block  to  the  spring  balance  and  pull  it  slowly  over 
the  board.  Read  the  scale  just  before  the  block  starts  and 
while  it  is  moving  uniformly.  Do  this  several  times  and  note 


36 


THE   MECHANICS   OF   SOLIDS 


the  readings.     Change  the  load  and  repeat.     Make  the  experi- 

ment again,  turning  the  block  upon  its  narrow  side. 

Conclusions.  —  Take  the  average  of  each  set  of  readings  and 

determine  the  relation  between  the  load  and  the  friction  of  the 

block. 

How  does  the  friction  on  starting  compare  with  that  after 

the  body  is  moving?     Does  a  change  in  the  area  of  the  side 

on  which  it  rests  change  the  friction? 

Suggestions.  —  Whenever  a  body  is  just  about  to  slide  down 

an  inclined  plane,  the  weight  0  W  (Fig.  22)  may  be  considered 

as  the  resultant  of  the 
pressure  upon  the  plane 
OP,  and  the  tendency,  OF, 
of  the  body  to  slide  down 
the  plane.  The  ratio 
between  these  two  com- 
ponents, OF:  OP,  is,  from 
similar  triangles,  the  same 
as  that  of  AC  :  CB.  If 
the  plane  is  hinged  at  B 

and  the  end   C  raised  until  the  body  begins  to  slide,  then 

a  measurement  of  AC  and  BC  will  give  the  coefficient  of  fric- 

tion; that  is,  /=  —  —  .     Find  the  coefficient  of  friction  by  this 


FIG.  22 


method. 


CB 
An 

The  ratio  —  =  tan  ABC. 


Hence,  we  may  say  that 


the  coefficient  of  friction  between  a  body  and  a  plane  surface 
is  equal  to  the  tangent  of  the  angle  between  the  surface  and 
a  horizontal  plane  when  the  body  begins  to  slide  downward. 


EXPEKIMENT    14 

The  Principle  of  Archimedes.  —  The  Lifting  Effect  of  a 
Liquid.     El.  of  Phys.,  p.  128 

(a)  Bodies  heavier  than  water. 

Apparatus.  — A  piece  of  aluminum  *  of  some  regular  shape, 
cylinder  or  prism;  an  overflow  can;  beaker;  balance  and 
weights ;  thread. 

Method.  —  Suspend  the  metal  block  from  one  arm  of  the 
balance  by  a  strong  thread  and  weigh  it.  Pour  water  into 
the  overflow  can  until  it  flows  out  the  spout  into  the  beaker. 
When  all  has  run  out  that  will,  throw  out  the  water  and  dry 
the  beaker.  Carefully  lower  the  metal  into  the  water  in  the 
overflow  can  and  catch  the  displaced  water  in  the  beaker. 
Find  the  weight  of  the  metal  when  it  is  submerged.  Weigh  the 
beaker  and  water.  Empty  the  beaker  and  dry  it  and  weigh 
again.  Tabulate  your  results  as  follows  :  — 

Weight  of  metal  in  air       .         .,       .         .        .        grams 

Weight  of  metal  in  water          ....        grams 

Weight  of  beaker  and  displaced  water      .        .        grams 

Weight  of  beaker  alone grams 

Find  the  relation  between  the  loss  of  weight  of  the  metal 
and  the  weight  of  the  displaced  water. 

Conclusions.  —  State  the  Principle  of  Archimedes  as  applied 
to  bodies  heavier  than  water. 

(&)  Bodies  lighter  than  water. 

Apparatus.  —  The  same  as  iii  (a)  except  that  the  aluminum 

1  Aluminum  is  suggested  as  a  good  metal  for  this  experiment  on  account 
of  its  lightness. 

37 


38 


LIQUIDS 


is  replaced  by  a  hardwood  block  that  has  been  paraffined  or 
coated  with  liquid  filler. 

Method.  —  Weigh  the  block  and  the  empty  beaker.  Pro- 
ceed with  the  overflow  can  as  in  (a).  Fasten  a  thread  to  the 
block  and  carefully  lower  it  upon  the  surface  of  the  water. 
Weigh  the  displaced  water.  Tabulate  your  results  as  follows  :  — 


Weight  of  block        ..   •     *  ; 
Weight  of  displaced  water 


grams 
grains 


Find  the  relation  between  these  two  weights. 

Conclusions.  —  State  the  Principle  of  Archimedes  as  applied 
to  bodies  lighter  than  water. 

Suggestion.  —  Shape  a  sheet  of  tin  so  that  it  will  float. 
Explain  why  it  floats. 


EXPERIMENT   15 

To  determine  the  Specific  Gravity  of  Solids  heavier  than 
Water.     El.  of  Phys.,  p.  133 

Apparatus.  —  A  pair  of  balances  with  one  arm  provided  with- 
a  suspension  hook;  beaker  of  water;  thread;  the  solids  to  be 

tested,  such  as  iron,  granite, 
sandstone,  aluminum,  etc. 

Method. —  Weigh  the 
solid  in  air,  suspending  it 
from  the  hook  with  a 
thread.  Fill  the  beaker 
two  thirds  full  of  water, 
distilled  if  possible  and  hav- 
.  ing  a  temperature  as  near 


4°  C.  as  possible.  Weigh 
FIG.  23.  the  solid  again  while  it  is 

submerged  in  the  water 

(Fig.  23),  being  careful  that  no  bubbles  of  air  are  attached 
to  the  solid.  Make  a  record  of  the  weights  as  follows :  — 


SPECIFIC    GRAVITY   OF   SOLIDS  39 


SUBSTANCE 

WEIGHT  IN  A  IB 

WEIGHT  IN  WATER 

Iron 

Granite 

Sandstone 

Conclusions.  —  Compute  the  specific  gravity  from  the  ex- 
pression 

g  _   Weight  of  solid  in  air 

Loss  of  weight  in  water 

Suggestions.  —  Why  should  distilled  water  be  used  ?  Why 
should  it  be  as  near  4°  C.  as  possible?  How  do  your  deter- 
minations of  specific  gravity  compare  with  those  of  the  same 
substances  as  given  in  the  table  ?  What  parts  of  the  experi- 
ment require  especial  care  ?  Find  out  whether  a  certain 
"  silver  "  spoon  is  solid  silver  or  plated. 

EXPERIMENT  16 

To  determine   the  Specific   Gravity  of  a  Body  lighter 
than  Water.     El.  of  Phys.,  p.  133 

Apparatus. — A  cake  of  wax;  aluminum  (or  other  metal) 
sinker ;  beaker  of  water ;  balance  and  weights. 

Method.' — Weigh  the  wax  in  the  air.  Call  this  weight  W. 
Weigh  the  sinker  in  the  water.  Call  this  weight  S.  Tie  the 
wax  and  sinker  together  and  weigh  them  in  water.  Call  this 
weight  W".  Compute  the  specific  gravity  of  the  wax  from  the 
expression 

=  W 

=  w+(S-  W") 

Conclusion. — The  reason  for  the  above  equation  may  be 
illustrated  as  follows:  Assume,  for  example,  that  a  piece  of 
paraffin  weighs  7.12  g.  ( IF)  in  air,  that  an  aluminum  sinker 


40  LIQUIDS 

weighs  6.28  g.  (S)  in  water,  and  that  the  two  when  tied 
together  and  weighed  in  water  weigh  5.4  g.  (  W").  Now  it  is 
evident  that  the  paraffin  displaces  enough  water,  not  only  to 
overcome  its  own  weight,  but  to  lift  6.28  g.  —  5.4  g.  =  0.88  g. 
of  the  weight  of  the  aluminum  sinker.  That  is,  the  paraffin 
displaces  7.12  g.  +  0.88  g.  =  8  g.  of  water.  Hence  the  specific 
gravity  of  the  paraffin  =  7.12  -H  8  =  0.89. 

State  the  Principle  of  Archimedes,  and  show  how  it  applies 
in  this  experiment. 

Suggestions.  — Find  the  specific  gravity  of  paraffin,  cork, 
and  a  tennis  ball. 


EXPERIMENT   17 

To  determine  the  Specific   Gravity  of  Liquids.     El.  of 
Phys.,  p.  134 

(a)   With  the  specific  gravity  flask. 

Apparatus.  —  A  specific  gravity  flask,  made  of  a  bottle  with 
an  ordinary  glass  stopper,  or  better  with  a  glass  stopper  having 
a  small  hole  through  it  lengthwise;  balance  and  weights; 
water;  and  the  liquid,  the  specific  gravity  of  which  is  to  be 
found,  —  alcohol,  for  example. 

Method.  —  Weigh  the  flask  empty.  Fill  it  with  water,  being 
careful  that  no  air  bubbles  remain  inside  when  the  stopper  is 
put  in.  Wipe  the  outside  of  the  flask  dry  and  weigh  it  again. 
Pour  out  the  water  and  dry  the  flask.  Fill  it  with  the  alcohol, 
taking  the  same  precautions  as  before,  and  weigh  again. 

Tabulate  your  results  as  follows :  — 

Weight  of  empty  flask  .         .        .        .        .        grams 

Weight  of  flask  and  water      ....        grams 

Weight  of  flask  and  alcohol  ....        grams 

Find  the  weight  of  the  water  and  of  the  alcohol  and  com- 
pute the  specific  gravity  of  the  alcohol. 


SPECIFIC    GRAVITY   OF   LIQUIDS  41 

Conclusion.  —  How  does  your  result  compare  with  the  spe- 
cific gravity  of  absolute  alcohol,  which  is  given  in  the  Smith- 
sonian Physical  Tables  as  0.789  at  20°  C.  ? 

(b)  With  the  specific  gravity  bulb. 

Apparatus.  —  A  specific  gravity  bulb  (see  Suggestions  below) ; 
balance  and  weights ;  beaker;  alcohol;  water. 

Method.  —  Weigh  the  bulb  in  air,  then  in  water,  and  then  in 
the  alcohol.  Compute  the  specific  gravity  of  the  alcohol  from 
the  equation, 

g  _  Loss  of  weight  in  alcohol 

Loss  of  weight  in  water 

Why  is  this  equation  correct  ? 

Suggestions.  — For  liquids  that  do  not  act  upon  it  a  piece  of 
aluminum  makes  a  good  specific  gravity  bulb.  A  glass  bulb 
can  be  made  as  follows :  Fuse  the  end  of  a  glass 
tube  a  half  inch  or  more  in  diameter  in  the  Bunsen 
flame  until  it  closes.  Let  the  tube  cool  aud  then 
heat  it  in  the  point  of  the  flame  at  a  distance  of 
two  inches  from  the  closed  end.  Draw  the  tube 
down  to  a  diameter  of  not  more  than  a  quarter 
of  an  inch  and  again  let  it  cool.  Pour  a  number 
of  shot  into  the  bulb  and  again  heat  at  the  narrow 
part.  Separate  the  bulb  and  draw  the  small  end 
into  the  form  of  a  hook  as  shown  in  Fig.  24. 

FIG.  24. 

(c)  With  Hare's  apparatus. 

Apparatus.  —  The  piece  of  apparatus  shown  in  Fig.  25.  A 
and  B  are  glass  tubes  joined  at  the  upper  ends  to  the  branches 
of  a  T  or  Y  tube.  The  other  end  of  the  Y  is  connected  to  a 
rubber  tube  R.  Under  the  lower  end  of  each  is  a  glass  dish, 
one  to  contain  water  and  the  other  alcohol.  (7  is  a  clamp,  and 
M a  meter  stick  between  the  tubes. 

Method.  —  Draw  the  air  from  the  tube  R  by  suction,  being 
careful  not  to  bring  the  column  of  liquid  in  either  glass  tube 


42 


LIQUIDS 


too  near  the-  top.  Close  R  with  the  clamp  and  take  readings 
both  of  the  height  of  liquid  in  the  tubes  and  of  that  in  the 
glass  dishes  at  the  lower  ends.  Loosen  the 
clamp  and  let  a  little  air  into  the  tube  R. 
Take  a  second  set  of  readings.  Remove 
the  clamp  from  the  tube  and  take  read- 
ings of  the  height  to  which  the  liquid 
stands  in  each  tube  due  to  capillarity. 

Conclusion.  —  Compute  the  specific  gravity 
of   the  alcohol  by  comparing  the  relative 

heights  of  the  water  column      m 

and  the  alcohol  column. 

Suggestions.  —  Bend  a  glass 
tube,  one  fourth  of  an  inch  or 
so  in   internal    diameter   and 
about  six  feet  long,  into  a  U 
form  with  the  branches  an  inch 
and  a  half  apart.    Fasten  them 
to  a  board  and  fasten  a  meter 
stick  to  the  board  between  the 
branches  of  the  U.     Pour  mer- 
cury into  one  branch  of  the 
tube  until   it   stands  at  least 
two  inches  high  in  each  branch. 
Pour  distilled  water  into  one  branch  until  it  stands 
about  twenty  inches  higher  than  the  top  of  the 
mercury  in  the  other  branch,  as  in  Fig.  26. 

Read  the  height  of  the  mercury  columns  in 
both  branches  and  the  height  of  the  water  column. 
Compute  the  specific  gravity  of  mercury  from  the 
data  found. 

Find  the  specific  gravity  of  alcohol  by  pouring 
alcohol  on  the  top  of  the  mercury  column  until  the  mercury 
column  is  at  the  same  height  in  both  branches.     Read  the 
heights   of  the   columns   and   compute   the    specific   gravity. 


FIG.  25. 


FIG.  26. 


SPECIFIC   GRAVITY   OF   LIQUIDS  43 

Gasoline  can  be  used  in  the  place  of  alcohol  if  preferred,  but 
great  care  must  be  taken  that  no  fire  cornes  near  it,  as  the 
vapor  is  very  inflammable. 

A  satisfactory  experiment  is  to  make  a  2  %  solution  of  salt 
in  water  and  find  its  specific  gravity.  Then  add  enough  salt 
to  make  a  4  %  solution,  and  so  on,  adding  salt  enough  to  make 
the  solution  2  %  greater  each  time.  Make  a  curve  to  show 
the  relation  between  the  percentage  of  salt  in  solution  and  the 
density. 


EXPERIMENT   18 

To  determine  the  Specific  Gravity  of  Air.     El.  of  Phys., 

p.  140 

Apparatus.  —  A  glass  bottle  of  three  or  four  liters  capacity  ; 
a  rubber  stopper  with  one  hole  ;  two  short  glass  tubes  ;  one 
short  and  one  long  rubber  tube  ;  a  clamp  ;  air  pump  ;  balance 
and  weights. 

Method.  —  Weigh  the  bottle  when  it  is  filled  with  air,  and 
when  the  stopper,  one  glass  tube,  short  rubber  tube,  and 
clamp  are  in  place. 

Pump  the  air  from  the  bottle,  close  the  clamp,  and  weigh 
again. 

Thrust  the  rubber  tube  under  water  and  open  the  clamp. 
Push  the  bottle  down  until  the  water  stands  at  the  same 
height  inside  and  outside.  Then  close  the  clamp,  wipe  dry 
the  outside  of  the  bottle  and  fittings,  and  weigh  the  bottle, 
fittings,  and  the  water  that  has  taken  the  place  of  the  air 
that  was  drawn  out  by  the  pump. 

Conclusion.  —  Call  the  first  weight  W,  the  second  W,  and 
the  third  W".  Compute  the  specific  gravity  of  the  air  from 
the  formula 

W  -W 


Show  why  this  will  give  the  specific  gravity.  Great  care 
should  be  exercised  in  taking  the  three  weights  exactly. 
Why  ?  Can  you  expect  to  get  very  accurate  results  by  this 
method  ? 

44 


BOYLE'S   LAW 


45 


Suggestion.  —  A  satisfactory  demonstration  of  the  weight  of 
air  may  be  made  as  follows :  Weigh  an  incandescent  lamp 
bulb  —  a  new  one  is  best,  though  one  with  a  broken  filament 
will  answer,  provided  no  leakage  has  taken  place.  Weigh  this 
as  carefully  as  possible.  With  a  blowpipe  direct  the  tip  of  a 
small  pointed  flame  from  a  Bunsen  burner  upon  the  side  of 
the  glass  (Fig.  27).  As  soon  as  it  gets  soft  a  small  hole  will 
be  blown  in  the  side  by  the  pressure  of  the  atmosphere  and 
as  it  is  from  without  inward  no 
part  of  the  lamp  will  be  lost. 
Weigh  the  lamp  carefully  again. 
Fill  the  bulb  entirely  full  oi 
water,  remove  all  water  from 
the  outside,  and  weigh  for  the 
third  time. 

From  the  data  thus  obtained 
find  the  specific  gravity  of  the 
air.  If  your  data  show  that 
there  was  some  gas  present  in 
the  lamp,  compute  the  amount 
present. 


EXPERIMENT  19 

To  find  the  Relation  between  the  Pressure  upon  a  Given 
Mass  of  Air  and  the  Resultant  Yolwine.  Boyle's  Law. 
El.  of  Phys.,  p.  150 

Apparatus.  —  The  usual  Boyle's  law  apparatus  (or  see  Sug- 
gestion below)  ;  mercury ;  barometer. 

Method.  —  Determine  the  pressure  of  the  atmosphere  by 
reading  the  barometer.  Set  up  the  apparatus  so  that  the  scale 
is  vertical.  Adjust  the  positions  of  the  arms  until  the  level  of 
the  mercury  is  the  same  in  both.  Take  the  reading  both  of 
the  mercury  surfaces  and  of  the  top  of  the  air  column,  and  com- 


46 


GASES 


pute  the  length  of  the  column  of  air.  Raise  the  open  tube  and 
take  a  reading  of  the  top  of  the  two  mercury  columns.  Repeat 
this  until  the  open  tube  is  raised  as  high  as  is  convenient.  Take 
a  reading  of  the  barometer  after  these  readings  are  finished 
and  compare  it  with  the  reading  taken  at  the  beginning. 

Conclusion.  —  The  closed  end  of  the  apparatus  should  be  of 
uniform  diameter,  hence  the  volume  of  the  inclosed  air  will 
be  proportional  to  the  length  above  the  end  of  the  mercury 
column. 

Compute  the  length  of  the  air  column  in  the  closed  tube  and 
the  corresponding  pressure  upon  it  for  each  set  of  readings, 
and  write  a  statement  of  the  law  that  shall  express  the  rela- 
tion between  the  volume  of  a  given  quantity  and 
the  pressure  upon  it. 

Suggestion.  —  A  convenient  form  of  apparatus 
can  be  made  by  taking  a  graduated  gas  burette 
with  a  stopcock  at  one  end  for  the  short  arm  of 
the  apparatus.  This  makes  it  easy  to  adjust  the 
amount  of  air  used  at  the  beginning,  and  to  find 
the  initial  volume.  A  straight  piece  of  glass 
tubing  can  be  connected  by  a  T  tube  and  two  short 
rubber  tubes  to  the  lower  end  of  the  burette,  and 
by  mounting  this  on  a  board  stand  as  shown  in 
Fig.  28,  and  fixing  a  meter  stick  in  place  in  a 
vertical  direction,  the  apparatus  is  completed. 

Slip  a  short  rubber  tube  and  clamp  on  the  end 
of  the  T.  Pour  mercury  in  the  long  tube  and 
bring  it  to  a  level  in  both  tubes  by  the  burette 
valve.  Regulate  the  amount  of  air  in  the  burette 
by  the  T  clamp.  Take  one  set  of  readings  by 
pouring  mercury  into  the  long  arm  a  little 
at  a  time,  and  a  second  set  by  drawing  mercury 
from  the  T  clamp  a  little  at  a  time.  Make  a 
graph  showing  the  relation  between  pressure  and 
FIG.  28.  volume. 


MANOMETER 


47 


EXPERIMENT   20 

To  measure  Gas  Pressure  by  the  Use  of  a  Manometer. 
EL  of  Phys.,  p.  151 

Apparatus.  —  An  open  manometer.  This  can  be  made  from 
a  6-ft.  piece  of  small  tubing  bent  and  mounted  on  a  support  as 
shown  in  Fig.  29. 

Method.  —  Pour  water  into  the  tube  at  the  end  A  until  it 
stands  at  about  the  point  G  in  both  tubes.  Couple  the  end  B, 
which  should  be  bent  away  from  the  support, 
to  the  gas  supply  by  a  rubber  tube  and  turn 
on  the  gas.  Read  the  water 
columns  in  both  tubes. 

Conclusions.  —  Compute  the 
pressure  of  the  gas  per  square 
centimeter  and  per  square 
inch. 

Suggestion.  —  Replace      the 

0  water  with  mercury,  and  rneas- 

|L  ure  the  pressure  in  a  football 
iiiil  by  coupling  it  both  to  a  force 
pump  and  to  the  manometer 
tube  at  B  by  a  T  tube.  If  the 
manometer  tube  is  not  long 
enough,  make  another  like  that 
in  Fig.  30,  using  a  strong  bottle 
for  a  mercury  tank. 

The  height  of  the  mercury  in 
the  tube  above  that  in  the  bottle 
FIG.  29.         is  a  measure  of  the  pressure.  FIG.  30. 


EXPERIMENT   21 


To  determine  the  Velocity  of  Sound  in  the  Laboratory 
and  to  measure  the  Wave  Length  of  Sound.  El.  of 
Phys.,  p.  178 

Apparatus.  —  A  tuning  fork  of  a  known  number  of  vibra- 
tions ;  a  cork  hammer;  resonance  tube  and  support,  as  shown 
in  Fig.  31.  This  form  of  tube  is  about  10  mm.  in  internal 

diameter  and  is  drawn  out 
and  connected  to  a  flexible 
rubber  tube  at  the  lower  end. 
This  is  in  turn  connected  to 
a  second  tube  which  is  fixed 
to  a  movable  wooden  arm. 
The  arm  moves  with  so  much 
friction  that  it  will  stay  in 
any  position.  Back  of  the 
tube  A  there  is  a  scale 
graduated  in  millimeters, 
the  zero  of  the  scale  being 
the  position  of  the  fork 
when  at  rest.  At  a  right 
angle  to  the  main  vertical 
support  is  another  which  is 
so  arranged  that  forks  of  various  lengths  may  be  held  in 
position. 

Method.  —  Place  the  tuning  fork  in  the  support  and  pour 
water  into  the  tube  until  it  is  at  least  half  full.  Move  B  from 
the  vertical  position  until  the  air  column  in  A  is  of  the  right 
length  to  give  the  maximum  resonance  to  the  fork.  This 

48 


FIG.  31. 


NUMBER   OF   VIBRATIONS  49 

position  can  be  found  accurately  by  moving  the  arm  B  back 
and  forth  through  the  required  position. 

Conclusions. — How  far  must  the  pulse  of  the  sound  wave 
given  out  by  the  fork  go  during  half  a  vibration  of  the  fork? 

What  is  the  relation  of  the  wave  length  of  the  fork  and  the 
length  of  the  air  column  ?  Compute  the  wave  length  of  the 
fork. 

For  accurate  work  a  correction  must  be  made  for  the  diam- 
eter of  the  tube.  This  practically  amounts  to  adding  the 
radius  of  the  tube  to  its  length. 

Compute  the  velocity  of  sound  by  the  formula  v  =  4  N(l  -f-  r). 
Explain  this  formula. 

For  comparison,  compute  the  velocity  of  sound  by  the  for- 
mula v  =  332 A  V 1  +  0.003665  t,  or  v  =  1090.5  Vl  +  0.003665 1. 
How  do  the  results  agree  ? 

Suggestions.  —  Use  a  fork  of  such  a  wave  length  that  you 
can  find  two  places  for  maximum  resonance.  Determine  the 
relation  between  the  wave  length  and  the  distance  between  the 
two  points. 

EXPERIMENT   22 

To  determine    the  Number  of  Vibrations  of  a  Tuning 
Fork.     El.  of  Phys.,  p.  184 

Apparatus.  —  A  cork  hammer  or  bass  viol  bow ;  the  special 
apparatus  shown  in  Fig.  32.  This  consists  of  a  rectangular 
board  with  a  groove  £  inch  deep  in  which  a  long  piece  of  glass, 
B,  slides  easily.  At  C  is  a  support  for  the  tuning  fork.  On 
D  a  movable  arm  A  is  clamped  to  support  the  pendulum. 

To  the  lower  side  of  the  pendulum  is  fastened  a  flexible 
bristle,  the  end  of  which  bears  lightly  upon  the  glass.  Near 
the  end  of  the  prong  of  the  tuning  fork  is  a  somewhat  stiffer 
bristle,  so  fastened  to  the  prong  by  a  piece  of  wax  that  it 
lightly  touches  the  glass  at  an  inclination  to  its  surface. 

PHYS.    LAB.   BOOK — 4 


50 


SOUND 


Method.  —  Adjust  the  support  of  the  pendulum  until  it  beats 
J  seconds.  This  must  be  accurately  determined  by  repeated 
trials.  Clean  the  glass  plate  and  lightly  smoke  its  surface  by 


FIG.  32. 

holding  it  over  the  flame  of  a  candle,  or  of  a  piece  of  burning 
camphor  (see  Suggestion  below).  Fix  the  fork  in  such  a  posi- 
tion that  the  two  bristles  are  very  near  to  each  other  in  the 
line  of  motion  of  the  glass  plate.  Put  both  the  fork  and 
the  pendulum  in  vibration  and  pull  the  glass  plate  along  in  the 
groove.  Remove  the  plate  and  count  the  number  of  vibrations 
of  the  fork  that  correspond  to  the  distance  between  two  marks 
of  the  pendulum,  taking  the  middle  of  each  mark  of  the  latter. 

Conclusion.  —  How  many  vibrations  does  the  fork  make  for 
each  vibration  of  the  pendulum  ?  How  many  per  second  ? 
What  is  the  pitch  of  the  fork  ? 

Suggestion. — The  following  is  a  much  cleaner  method  of 
taking  the  trace.  Cover  the  glass  plate  wTith  a  thin  coat  of 
kerosene,  or  vaseline,  by  rubbing  the  plate  with  a  cloth  on 
which  you  have  put  a  few  drops  of  the  oil.  Then  sift  a 
little  flour  or  cornstarch  on  the  plate  through  a  sieve  made  by 
stretching  a  piece  of  thin  muslin  between  two  embroidery 
hoops.  Jar  the  plate  slightly  to  remove  the  loose  flour,  and 
there  will  be  left  a  coating  that  will  give  a  good  trace. 


EXPERIMENT   23 

Testing  the  Fixed  Points  of  a  TJiermometer.     El.  of 
Phys.,  p.  I'll 

(a)  To  test  the  freezing  point. 

Apparatus.  —  Thermometer ;  a  dish,  perforated  at  the  bottom, 
such  as  a  small  flowerpot  or  a  funnel;  cracked  ice  or  snow. 

Method. — Place  the  bulb  of  the  thermometer  in  the  dish 
and  surround  it  completely  with  cracked  ice,  covering  the 
stein  almost  to  the  freezing  point,  0°  C.  or  32°  F.  Let  it  stand 
in  the  melting  ice  until  there  is  no  change  in  the  position  of 
the  top  of  the  mercury  column,  and  then  take  a  .reading  to 
the  tenth  of  a  degree. 

Conclusion.  —  Why  should  the  ice  be  melting  when  the 
reading  is  taken?  Make  a  record  of  the  reading  of  the  ther- 
mometer and  observe  whether,  the  error  is  plus  or  minus. 

(5)  To  test  the  boiling  point  of  a  tJiermometer. 

Apparatus.  —  A  boiler  or  a  large  Florence  flask;  a  stopper 
with  two  holes ;  thermometer  to  be  tested ;  a  standard  ther- 
mometer; Bunsen  burner  and  tripod  ;  barometer.. 

Method. — Compare  the  reading  of  the  thermometer  to  be 
tested  with  a  standard  thermometer.  Put  such  a  quantity  of 
water  in  the  flask  that  when  the  bulb  of  the  thermometer, 
which  has  been  pushed  through  one  hole  in  the  stopper,  is 
about  an  inch  above  the  water,  the  boiling  point,  100°  C.  or 
212°  F.,  shall  be  just  above  the  stopper.  The  other  hole  of 
the  stopper  should  be  left  open  for  the  free  escape  of  the 
steam. 

Place  the  flask  on  the  tripod  and  apply  the  heat  of  the 

51 


52 


HEAT 


Bunsen  burner.  When  the  steam  has  been  coming  from  the 
flask  for  about  five  minutes  and  there  is  no  further  change  in 
the  mercury  column,  take  a  reading  of  the  boiling  point. 
Note  the  reading  of  the  barometer.  Remove  the  thermometer 
from  the  flask  and  take  simultaneous  readings  of  the  tempera- 
ture of  the  air  with  both  the  thermometer  used  in  the  experi- 
ment and  the  standard  thermometer. 

Conclusions. — From  the  reading  of  the  boiling  point  and 
that  of  the  freezing  point  determine  what  correction  per  degree 
must  be  made  for  accurate  work.  As  the  barometric  pressure 
affects  the  temperature  of  the  boiling  point,  it  may  be  necessary 
to  make  a  correction  in  the  reading  on  that  account,  though  the 
change  will  probably  be  very  slight.  In  order  that  the  boiling 
point  shall  be  100°  C.  or  212°  F.,  the  barometric  pressure  must 
be  760  mm.  A  change  of  27  mm.  in  the  reading  of  the 
barometer  when  the  temperature  is 
near  100°  C.  causes  a  change  of  1°  in 
the  centigrade  reading,  and  a  change 
of  15  mm.  causes  a  change  of  1°  in 
the  Fahrenheit  reading. 

How  soon  after  a  thermometer  has 
been  raised  to  100°  C.  would  you  use 
it  for  accurate  work  at  ordinary  air 
temperatures? 

Suggestion.  —  The  fact  that  there 
is  a  rise  in  temperature  with  an 
increase  of  pressure  can  be  shown  by 
making  use  of  a  boiler  like  that 
shown  in  Fig.  33.  The  pressure  can 
be  increased  by  partly  stopping  up 
the  exit  of  the  steam  pipe.  The 
amount  of  the  pressure  can  be  found 
by  reading  the  difference  in  the 
height  of  the  mercury  in  the  sides 
FIG.  33.  of  the  U  tube  m. 


COEFFICIENT  OF  LINEAR  EXPANSION 


53 


EXPERIMENT   24 

To  find  the  Coefficient  of  Linear  Expansion  of  Solids. 
El.  of  Phys.,  p.  228 

Apparatus. — Linear  expansion  apparatus  (Fig.  34);  boiler  or 
flask;  delivery  tube;  Bunsen  burner;  meter  scale;  a  brass, 
iron,  or  aluminum  tube  to  be  tested. 

Method. — Pass  the  end  of  the  screw  hook  which  forms  the 
fixed  point  A  (Fig.  34)  through  a  hole  near  one  end  of  the 
metal  tube,  and  adjust  the  short  end  of  the  lever  in  a  hole  in 


FIG.  34. 

the  upper  side  of  the  tube  at  B.  Measure  accurately  the 
length  of  the  tube  between  A  and  B,  and  the  length  of  each 
lever  arm. 

Let  the  apparatus  stand  for  some  time  at  the  temperature 
of  the  room,  and  then  take  a  reading  of  the  long  lever  arm. 
Call  this  reading  the  zero  reading.  Take  a  reading  of  a 
thermometer  near  the  apparatus. 

Fill  the  boiler  about  a  third  full  of  water,  connect  with  the 
delivery  tube,  leaving  the  end  of  the  tube  open,  and  heat  over 
a  Bunsen  burner.  When  the  water  boils,  connect  the  delivery 
tube  to  the  metal  tube,  being  careful  not  to  disturb  the  position 


54  HEAT 

of  the  pointer.  When  steam  has  been  coming  freely  from  the 
tube  for  some  minutes  and  there  is  no  further  movement  of  the 
pointer,  take  a  reading  of  the  position  of  the  pointer.  Record 
your  results  as  follows :  — 

Initial  length  of  tube     .          .;.'.     .         .  ....  —cm. 

Length  of  long  lever  arm    ....>,       .          .  .  '  . ..  ..•  —cm. 

Length  of  short  lever  arm       .      ..,...,       .  .        ,  .  —  cm. 

Initial  reading  of  long  lever  arm     .          .  .  i  •'  ;  —cm. 

Final  reading  of  long  lever  arm       .          .  .          .  —cm. 

Initial  temperature        .         .         •«         .  .  "•:'.•  -°  C. 

Final  temperature          .         .      ;  V  :  ••*•'.  •  ;•  100°  C. 

Conclusions.  —  From  the  results  obtained  compute :  — 
The  change  of  temperature  of  the  tube. 

The  expansion  of  the  tube,  or  the  distance  the  short  lever 
arm  moved,  from  the  proportion 

Length  of  long  lever  arm  :  Length  of  short  lever  arm 

=  Distance  the  long  arm  moved  :  Distance  the  short  arm  moved. 

The  expansion  of  the  tube  for  1°  change  of  temperature. 

The  expansion  of  1  cm.  of  the  tube  for  1°  change  of  tempera- 
ture, or  the  coefficient  of  linear  expansion. 

Suggestion.  —  The  loss  of  heat  by  radiation  from  the  tube 
can  be  kept  down  by  surrounding  the  greater  part  of  the  tube 
with  a  glass  tube. 


EXPERIMENT   25 

To  determine  the  Dew-Point  and  Relative  Humidity. 
El.  of  Phys.,  pp.  239-240 

Apparatus. — A  small  nickel-plated  calorimeter;  ice  water; 
thick  pad  of  woolen  or  felt ;  thermometer. 

Method.  —  Place  the  felt  pad  upon  a  table  and  put  the  calo- 
rimeter upon  it.  Pour  water,  at  the  room  temperature,  into 
the  calorimeter  until  it  is  half  full.  Insert  the  thermometer 


DEW-POINT   AND   RELATIVE   HUMIDITY 


55 


and  pour  ice  water  into  the  calorimeter,  stirring  constantly 
with  the  thermometer.  Take  the  temperature  as  soon  as  mist 
is  seen  to  form  on  the  polished  surface  of  the  calorimeter. 
Stop  adding  ice  water  and  watch  the  surface  to  note  the  dis- 
appearance of  the  mist.  Take  the  temperature  at  the  time  it 
disappears. 

Conclusion.  —  Compute  the  dew-point  by  averaging  the  two 
temperatures. 

This  point  found,  it  will  be  possible  to  find  the  relative 
humidity  if  we  know  the  number  of  grams  of  water  vapor 
that  saturated  air  contains  per  cubic  meter  at  different  temper- 
atures. This  is  shown  in  the  table  on  page  239  of  the  Elements 
of  Physics.  Find  the  temperature  of  the  air.  Take  from  the 
table  the  amount  of  vapor  that  the  air  will 
hold  at  this  temperature  and  at  that  of  the 
dew-point  which  you  have  just  found,  and 
find  the  relative  humidity  by  dividing  the 
amount  at  the  temperature  of  the  dew-point 
by  the  amount  at  the  temperature  of  the  air. 

Suggestions.  —  A  simple  form  of  apparatus 
(Fig.  35)  for  the  determination  of  the  dew- 
point  can  be  made  as  follows:  Blacken  an 
inch  of  the  lower  end  of  a  large  test  tube  by 
painting  it  with  asphalt  varnish.  Pour  a 
little  ether  into  the  tube.  Insert  a  ther- 
mometer and  take  its  temperature.  Insert  a 
glass  tube  into  the  test  tube  and  connect  it  to 
a  rubber  tube  two  feet  long,  having  a  glas"s 
mouthpiece  at  the  other  end.  Blow  through 
this  tube  gently  and  notice  that  as  the  ether  evaporates  more 
rapidly  its  temperature  falls.  Keep  blowing  until  a  mist  forms 
at  the  lower  part  of  the  tube.  Take  the  temperature  as  soon 
as  the  mist  begins  to  form.  Stop  blowing  and  take  the  tem- 
perature again  just  as  the  mist  disappears.  Repeat  and  take 
the  average  of  several  trials. 


FIG.  35. 


56  HEAT 

EXPERIMENT   26 

To  find  the  Law  of  Heat  Exchange  ~by  the  Method  of 
Mixtures.     El.  of  Phys.,  p.  248 

Apparatus.  —  Three  beakers  —  two  of  the  same  size  ;  balance 
and  weights,  or  graduate ;  Bunsen  burner  and  tripod. 

Method.  —  Place  in  one  of  the  two  similar  beakers  300  g.  of 
water.  Place  this  on  the  tripod  and  heat  the  water  quite  hot 
with  the  Bunsen  burner.  Pour  into  the  other  beaker  300  g.  of 
cold  water.  Take  the  temperature  of  the  water  in  both  beak- 
ers and  quickly  pour  their  contents  into  the  third.  Take  the 
temperature  of  the  mixture.  Consider  this  experiment  pre- 
liminary, and  repeat,  bringing  the  third  beaker  to  about  the 
temperature  of  the  mixture  in  the  first  experiment  before  you 
pour  in  the  contents  of  the  other  two,  so  that  none  of  the  heat 
may  be  lost  in  warming  the  beaker  that  is  to  contain  the  mix- 
ture. 

Conclusions.  —  Is  the  resulting  temperature  a  mean  between 
the  other  two?  Compute  the  number  of  calories  of  heat  given 
off  by  the  hot  water  =  (mass  in  grams  x  change  in  tempera- 
ture in  centigrade  degrees) ;  also  number  of  calories  of  heat 
absorbed  by  the  cold  water  —  (mass  in  grams  x  change  in 
temperature  in  centigrade  degrees).  What  is  the  relation  be- 
tween the  heat  given  out  and  the  heat  absorbed  ? 

EXPERIMENT   27 

To  find  the  Specific  Heat  of  a  Solid.     El.  of  Phys.,  p.  248 

• 

Apparatus.  —  A  Bunsen  burner ;  water  boiler ;  copper  calo- 
rimeter; thermometer;  balance  and  weights;  sheet  of  alumi- 
num, the  specific  heat  of  which  is  to  be  measured;  a  thick  felt 
pad. 

Method.  —  Roll  the  metal  sheet  into  an  open  spiral  roll  with 
about  a  quarter  of  an  inch  between  the  surfaces.  Attach  a 


SPECIFIC    HEAT  OF  A   SOLID  57 

fine  piece  of  copper  wire  to  one  corner  of  the  roll  for  a  handle. 
Weigh  the  roll.  Weigh  the  empty  calorimeter.  Place  suffi- 
cient water  in  the  calorimeter  to  cover  the  roll,  and  weigh. 
Set  the  calorimeter  upon  the  felt  pad.  Place  the  roll  in  the 
boiler  and  cover  with  water.  Heat  over  the  Bunsen  burner. 
W7hen  the  water  has  been  boiling  for  some  time,  take  the  tem- 
perature of  the  water  in  the  calorimeter  and  transfer  the  roll 
from  the  boiling  water  to  the  calorimeter  as  quickly  as  possible. 
Stir  the  water  in  the  calorimeter  and  take  its  temperature  near 
the  top  of  the  water,  near  the  bottom,  and  at  different  points 
near  the  sides  of  the  roll.  If  these  readings  differ,  stir  again 
and  repeat,  taking  the  highest  uniform  temperature  as  the 
reading. 

Record  your  results  as  follows :  — 

Mass  of  metal g. 

Mass  of  calorimeter *  .'        .         .  g. 

Mass  of  calorimeter  and  water g. 

Mass  of  water g. 

Temperature  of  water g. 

Temperature  of  metal g. 

Resulting  temperature  of  water  and  metal      .         .        .  g. 

Conclusion.  —  Compute:  (1)  Change  in  temperature  of  water. 

(2)  Change  in  temperature  of  metal. 

(3)  Heat  absorbed  by  water.     This  may  be  represented  by 
the  expression  mts;  that  is,  the  product  of  the  mass  in  grams, 
the  change  of  temperature  in  centigrade  degrees,  and  the  spe- 
cific heat  of  the  substance. 

(4)  Heat  given  out  by  the  metal.     This  may  be  expressed 
as  m't's'. 

(5)  The  specific  heat  of  the  aluminum,  from  the  equation 

Heat  absorbed  by  water  =  heat  given  out  by  aluminum 
or  mts  =  m't's'. 

(6)  Per  cent  of  error,  found  by  comparison  of  this  specific 
heat  with  0.214,  the  value  given  in  the  table,  El.  of  Phys.,  p.  248. 


58  HEAT 

Suggestions.  —  In  making  accurate  determinations,  allowance 
must  be  made  for  the  heat  absorbed  by  the  calorimeter,  which 
may  be  done  by  finding  its  water  equivalent.  This  is  the  prod- 
uct of  its  mass  in  grams  by  its  specific  heat,  and  is  added  to 
the  mass  of  the  water  as  explained  in  El.  of  Phys.,  pp.  249, 
250. 

On  account  of  its  small  specific  heat  it  is  better  to  use  a 
copper  calorimeter  in  these  experiments  than  one  of  glass.  If 
its  surface  is  burnished,  the  loss  by  radiation  is  also  less. 

A  convenient  stirrer  is  made  by  cutting  two  slits  in  a  thin 
sheet  of  copper  about  1x1^  inches  and  slipping  it  on  to  the 
thermometer  bulb.  It  will  be  well  to  test  the  temperature  of 
the  boiling  water  with  a  second  thermometer. 

EXPERIMENT   28 

To  determine  the  Heat  of  Fusion  of  Ice.     El.  of  Phys., 

p.  249 

Apparatus.  —  Calorimeter ;  ice  bag  of  heavy  cotton  or  can- 
vas; thermometer;  Bimsen  burner;  tripod;  beaker;  balance 
and  weights;  a  mallet. 

Method.  —  Pour  into  the  beaker  about  400  cc.  of  water  and 
heat  it  over  the  Bunsen  flame.  Weigh  the  calorimeter  and  call 
the  mass  M.  Put  about  100  cc.  of  ice  in  the  ice  bag  and 
break  it  into  fine  pieces  with  the  mallet.  When  the  water 
in  the  beaker  is  heated  to  about  80°  C.,  pour  about  200  cc.  into 
the  calorimeter  and  weigh  again,  calling  the  mass  M' .  Read 
the  temperature  of  the  water  in  centigrade  degrees  and  call 
it  T.  Take  the  crushed  ice  from  the  bag  and  put  it  into  the 
calorimeter,  being  careful  that  the  ice  is  dry. 

Stir  the  water  and  ice  with  the  thermometer  and  read  the 
temperature,  in  centigrade  degrees,  as  soon  as  the  ice  is  all 
melted.  Call  this  temperature  T'.  Weigh  the  calorimeter  and 
contents,  and  call  the  mass  M ". 


HEAT  OF  VAPORIZATION   OF   WATER  59 

Conclusion.  —  Compute  the  heat  of  fusion  of  ice,  i.e.  the 
number  of  calories  required  to  melt  one  gram  of  ice,  from  the 
following  data  :  — 

Mass  of  calorimeter    .         .        .         ...         .         .  M 

Mass  of  water     .          .......  {M1  —  M} 

Mass  of  ice          ........  (M"  -  M') 

Change  of  temperature  of  water  and  calorimeter        .  (T—T'} 

Change  of  temperature  of  the  water  from  melted  ice  .  T 

Heat  given  out  by  hot  water       .....  (M'  —  M}(T—T} 

Heat  given  out  by  the  calorimeter      .         .         .         .  M(  T—  T')  C 

In  this  expression  C  is  the  specific  heat  of  the  calorimeter  metal. 

Heat  absorbed  by  the  ice  in  melting   .         .         .  F(M"  —  M} 

In  this  expression  F  is  the  heat  of  fusion. 

Heat  absorbed  in  heating  the  resulting  water  to  T'°  C.  (M"  —  M')  T1 


Hence    F  =  (M'-M}(T-  T)  +  M(T-  T'}C-(M"-M>)  T' 

M1  -M' 

Suggestions.  —  It  will  be  well  to  consider  the  first  experi- 
ment as  a  preliminary  trial,  and  so  to  adjust  the  amounts  of 
water  and  ice  taken  in  the  second  that  the  final  temperature 
will  be  nearly  that  of  the  room.  This  will  make  the  losses  due 
to  radiation  so  small  that  they  may  be  disregarded.  If  the 
water  in  the  calorimeter  cools  too  much  while  it  is  being 
weighed,  it  should  be  set  on  the  tripod  and  again  brought  to 
the  proper  temperature.  Aluminum  is  chosen  for  the  test  on 
account  of  its  high  specific  heat.  Shot  can  be  used,  but  the 
low  specific  heat  of  lead  is  apt  to  make  the  result  unsatisfactory. 


EXPERIMENT   29 

To  determine  the  Heat  of  Vaporization  of  Water.     El.  of 
Phys.,  p.  250 

Apparatus.  —  Bunsen  burner  and  tripod  ;  steam  boiler ;  con- 
densation trap ;  calorimeter ;  non-conducting  pad ;  cracked  ice  ; 
thermometer;  a  screen. 


60 


HEAT 


Method.  —  Fill  the  boiler  about  half  full  of  water,  place  it 
upon  the  tripod,  connect  it  with  the  trap  and  delivery  tube,  as 
in  Fig.  36,  and  light  the  Bunsen  flame.  Set  up  the  screen 
between  the  Bunsen  flame  and  the  calorimeter  so  as  to  keep  off 
the  radiant  heat  as  much  as  possible.  It  will  be  well  to  cover 
the  steam  tubes  with  felt  or  cotton.  Cool  some  water  with 
cracked  ice  until  it  is  about  15°  below  the  temperature  of  the 


FIG.  36. 


room.  Weigh  the  calorimeter  and  pour  it  about  two  thirds 
full  of  the  cooled  water.  Weigh  the  calorimeter  with  the 
water  in  it.  When  the  steam  comes  freely  from  the  delivery 
tube  without  any  condensed  water,  take  the  temperature  of 
the  water  in  the  calorimeter  and  place  the  end  of  the  delivery 
tube  about  2  in.  below  the  surface  of  the  water.  Stir  the 
water  in  the  calorimeter  with  the  thermometer,  and  when  it  is 
as  much  above  the  temperature  of  the  room  as  the  cooled 
water  was  below,  remove  the  end  of  the  delivery  tube,  stir 
the  water  with  the  thermometer,  and  take  its  highest  tempera- 
ture. Weigh  the  calorimeter  and  contents. 


VAPOR  PRESSURE  61 

Conclusion.  —  Record  your  results  as  follows :  — 

Mass  of  calorimeter       ........  

Mass  of  calorimeter  and  water        ......  

Mass  of  calorimeter,  water,  and  condensed  steam      .          .          .  

Temperature  of  cooled  water  and  calorimeter  .          .          . 

Final  temperature  of  calorimeter,  water,  and  condensed  steam    .  • 

Compute :  — 

Mass  of  water  before  adding  steam  ......     

Mass  of  steam  added      ........     

Change  in  temperature  of  calorimeter  and  water        .          .          .  — 

Change  in  temperature  of  steam     ......     

Heat  absorbed  by  water         .......     

Heat  absorbed  by  calorimeter          ...          .          .          .          .     

Heat  given  out  in  changing  steam  to  water       ....     

Heat  given  out  by  changing  temperature  of  condensed  steam       .     

From  these  data  find  the  calories  of  heat  given  out  by  one 
gram  of  steam  at  100°  in  condensing  to  water  at  100°. 

EXPERIMENT  30 

To  find  the  Pressure  of  Saturated  Ether  Vapor  at  Dif- 
ferent Temperatures.     El.  of  Phys.,  pp.  253-255 

Apparatus.  —  Glass  tubing;  mercury;  Bunsen  burner;  a 
tall  glass  beaker;  metric  scale;  thermometer;  ether. 

Method.  —  Bend  a  piece  of  small  glass  tubing  into  the  form 
of  a  U  tube  with  one  arm  twice  as  long  as  the  other  (Fig.  37),  — 
15  cm.  for  the  short  arm  and  30  cm.  for  the  long  arm  are  suit- 
able lengths, — and  close  the  short  arm.  Mark  off  a  scale  along 
a  narrow  board  and  wire  the  tube  and  thermometer  to  it.  Fill 
the  tube  to  within  a  quarter  of  an  inch  of  the  top  of  the  open 
end  with  mercury,  by  pouring  mercury  into  the  long  arm  and 
manipulating  the  tube  until  the  air  passes  out  of  the  short 
tube  and  mercury  takes  its  place.  Put  in  ether  enough  to  fill 
it  completely,  and  so  manipulate  the  tube  as  to  transfer  the 
ether  to  the  upper  end  of  the  tube  at  A.  Push  an  iron  rod  or 


62 


HEAT 


wire  into  the  open  end  of  the  long  arm  and  force  out  some  of 
the  mercury  until  it  stands  at  the  same  height  in  both  arms. 
Pour  water  into  the  beaker  to  such  a  depth  that  it 
will  cover  the  short  end  of  the  tube  when  it  is 
placed  in  the  beaker  and  heat  it  to  about  35°  C. 
(Caution:  Be  careful  to  have  no  open  flame  come 
near  either  the  ether  or  its  vapor,  since  both  are 
very  inflammable.)  Lower  the  tube  into  the  water 
carefully  and  note  the  result.  The  ether  will  prob- 
ably not  vaporize  at  that  temperature.  Remove 
the  tube  and  add  a  small  quantity  of  hot  water. 
Insert  the  tube  again  and  observe.  Keep  adding 
hot  water  until  the  ether  vaporizes,  being  careful 
that  this  does  not  take  place  so  quickly  as  to 
force  the  mercury  from  the  end  of  the  long  arm. 
When  the  water  is  at  such  a  temperature  that 
the  mercury  will  stand  nearly  at  the  top  of  the 
long  arm,  take  a  reading  of  the  temperatures  and 
pressure.  Take  frequent  readings  as  the  water 
cools  to  such  a  temperature  that  all  the  vapor  has  liquefied. 

Conclusion.  —  Make   a  graph   that   shall  show  the   relation 
between  the  temperature  and  pressure  of  ether  vapor. 

Suggestions.  —  Find  in  this  experiment  the  boiling  point  of 
ether. 

Make  the  same  experiment  with  alcohol  and  note  the  differ- 
ence. 

Why  is  the  experiment  difficult  to  make  with  water  ? 
Is  there  a  suggestion  in  this  experiment  of  the  pressure  of 
superheated  steam  ? 


FIG.  37. 


EXPERIMENT  31 
Lines  of  Force  in  a  Magnetic  Field.     El.  of  Phys.,  p.  265 

Apparatus.  —  Bar  and  horseshoe  magnets;  plate  of  window 
glass  ;  sheet  of  white  unruled  paper;  wooden  strips  of  the  same 
thickness  as  the  magnets;  iron  filings  and  a  sieve  made  by 
stretching  a  piece  of  cheesecloth  over  a  pair  of  embroidery 
rings. 

Method.  —  Place  a  bar  magnet  upon  the  table  and  around  it 
place  several  wooden  strips  to  steady  the  glass  plate.  Lay  over 
the  magnet  the  sheet  of  paper  and  upon  this  place  the  plate  of 
glass.  Sift  iron  filings  evenly  over  the  plate.  Jar  the  plate 
by  rapping  it  on  the  edge  with  a  lead  pencil  until  the  filings 
show  by  their  position  the  direction  of  the  lines  of  force;  and 
by  their  arrangement  the 


_  ________ 


a 


relative    intensity    of    the^ 
magnetic  field. 

Make  similar  studies  of 
the  lines  of  force  around 
the  end  of  a  bar  magnet: 
in  the  space  between  the 


end  of  a  long  bar  magnet  FIG  3g 

and  a  short  magnet  placed 

as  in  (a),  Fig.  38;  in  the  space  between  the  end  of  a  bar 
magnet  and  a  horseshoe  magnet  placed  as  in  (6) ;  and  in  the 
space  between  the  end  of  a  bar  magnet  and  a  piece  of  soft 
iron  of  the  same  width  and  thickness  as  the  magnet,  as  in  (c). 
Conclusion.  —  Make  drawings  of  the  fields  examined  by  rep- 
resenting the  magnets  by  full  lines,  and  the  lines  of  force  by 
fine  dotted  lines.  Assume  the  direction  of  the  lines  of  force  to 


64  MAGNETISM 

be  that  in  which  the  north  pole  of  a  needle  would  be  propelled, 
i.e.  from  the  +  pole  of  the  magnet  through  the  space  around 
the  magnet  to  the  —  pole,  and  indicate  it  by  an  arrow  point  on 
the  dotted  lines. 

In  what  direction  do  the  lines  of  force  pass  within  the  magnet  ? 

State  the  law  of  mutual  action  in  magnets  and  give  examples 
of  its  evidence  from  your  drawings. 

Suggestion.  — The  best  method  of  making  permanent  records 
of  these  curves  is  as  follows :  Study  them  until  an  example 


FIG.  39. 

is  found  that  is  wanted.  Then  go  into  a  photographic  dark 
room  and,  using  a  photographic  plate  for  the  support,  place  it, 
film  side  up,  on  the  magnet.  Sift  on  the  filings,  rap  the  plate 
until  they  are  in  the  right  position,  then  expose  the  plate  by 
burning  a  match  a  foot  or  more  above  the  plate.  Develop  the 
plate  in  the  usual  way  and  thus  obtain  a  negative  from  which 
any  number  of  prints  similar  to  Fig.  39  can  be  made. 

Blue  print  paper  can  be  used  on  which  to  sift  the  filings. 
When   arranged   satisfactorily,  the  paper  can  be  exposed  by 


LINES   OF   FORCE    IN   A   MAGNETIC   FIELD 


65 


placing  in  the  sunlight  and  the  developing  can  be  done  in 
water.  This,  however,  gives  white  lines  on  a  blue  ground, 
that  is,  a  negative. 


EXPERIMENT   32 

Mapping  Lines  of  Force  in  a  Magnetic  Field.    El.  of  Phys., 

p.  266 

The  force  that  determines  the  direction  in  which  a  compass 
needle  points  is  the  resultant  of  all  the  magnetic  forces  that 
act  upon  the  needle.  As  it  is  generally  necessary  to  make 
magnetic  experiments  in  the  magnetic  field  of  the  earth,  the 
effect  of  that  field  must  be  taken  into  the  account.  The  effect 
of  this  field  can  be  eliminated  if  the  precaution  is  taken  so  to 
arrange  the  position  of  the  apparatus  that  the  magnetic  needle 
is  in  the  magnetic  meridian  at  the  time  the  reading  is  taken. 

To  map  out  the  direction  of  the  lines  of  force  around  a 
bar  magnet. 

Apparatus. — A  bar  magnet;  small  compass  not  more  than 
half  an  inch  in  diameter;  a  drawing  board  twice  as  long  as 
the  magnet;  a  sheet  of 
drawing  paper  pinned  to 
the  board. 

Method.  —  Lay  the 
magnet  in  the  middle  of 
the  paper  with  its  length 
parallel  to  one  side. 
Place  the  compass  at 
one  corner  of  the  magnet 
and  turn  the  board  until  Fio.  40. 

the  needle  is  in  the  mag- 
netic meridian.     With  a  sharp  pencil   place  a  small  dot  at 
the  -f-  end   of  the  needle   just  outside  of   the  compass  case 
(Fig.  40).     Move  the  compass  along  until  the  dot  is  just  at  the 

PHYS.   LAB.   1JOOK 5 


66  MAGNETISM 

edge  of  the  case  and  nearest  the  —  end  of  the  needle,  turning 
the  board  until  the  needle  is  again  in  the  meridian.  Repeat  the 
process  until  the  compass  is  brought  back  to  some  part  of 
the  magnet.  Then  the  curved  row  of  dots  will  mark  out  the 
path  of  one  of  the  lines  of  force  of  the  bar  magnet.  Make  a 
second  curve,  beginning  at  a  short  distance  from  the  first. 
Repeat  until  the  whole  of  the  space  around  the  magnet  is 
mapped.  Draw  a  figure  of  the  magnet  by  passing  a  pencil 
point  about  it  before  you  remove  it  from  the  paper,  and  mark 
the  -f-  and  —  ends. 

Conclusion.  —  Name  this  drawing,  Lines  of  Force  around  a 
Bar  Magnet  —  Earth's  Field  Neutralized. 

Suggestion. — Make  the  same  experiment,  placing  the  magnet 
in  the  east  and  west  direction  and  not  changing  the  position  of 
the  board.  In  this  case  the  direction  of  the  lines  mapped  out 
will  show  the  influence  of  the  earth's  magnetism.  Call  this 
drawing,  Lines  of  Force  around  a  Bar  Magnet  —  Earth's  Field 
not  Neutralized.  Compare  the  two  drawings  and  explain. 

EXPERIMENT  33 
To  make  a  Permanent  Magnet.     El.  of  Phys.,  pp.  263-274 

(a)   By  the  use  of  a  bar  magnet. 

Apparatus.  —  A  knitting  needle ;  bar  magnet ;  small  com- 
pass. 

Method.  —  Grind  or  file  off  one  end  of  a  knitting  needle  to 
a  flat  end.  Lay  the  needle  on  the  table  and  stroke  it  once 
from  the  middle  to  the  marked  end  with  the  +  end  of  a  bar 
magnet.  Reverse  the  needle  and  stroke  the  unmarked  end  once 
with  the  —  end  of  the  magnet:  Place  the  compass  on  the 
table  and  bring  the  marked  end  of  the  knitting  needle  to  the 
west  side  of  the  compass  with  the  needle  pointing  directly 
toward  it.  Mark  the  position  of  the  marked  end  of  the  needle 
when  the  compass  needle  is  deflected  ten  or  fifteen  degrees. 


TO   MAKE   A   PERMANENT   MAGNET  67 

Turn  the  needle  end  for  end  and  bring  the  pointed  end  into  the 
first  position  of  the  marked  end.  Note  the  direction  and  the 
amount  of  the  deflection.  Stroke  each  end  of  the  needle  as 
before  and  test  the  deflection.  Repeat  until  the  deflection  of 
the  compass  needle  has  reached  a  maximum,  i.e.  until  the 
magnetism  of  the  knitting  needle  is  no  longer  increased  by  the 
process. 

Conclusion.  —  State  in  your  notes  the  polarization  shown  by 
each  end  of  the  knitting  needle  and  the  effect  that  each  stroke 
had  upon  the  extent  of  the  deflection. 

(b)   By  the  use  of  a  horseshoe  magnet  without  stroking. 

Apparatus. —  Sewing  needle  ;  horseshoe  magnet;  compass; 
a  thin  card. 

Method.  —  Lay  the  card  upon  the  poles  of  the  horseshoe 
magnet  and  place  the  needle  upon  it  with  the  eye  end  over  the 
+  pole  and  the  point  over  the  —  pole.  Rap  the  needle  sharply 
with  a  pencil  or  a  knife  handle.  Test  it  for  polarity  with  the 
compass.  Repeat  and  test  again  until  you  have  the  maximum 
deflection. 

Conclusions.  —  State  the  polarity  of  the  needle  and  explain 
why  it  is  magnetized.  Has  your  needle  been  saturated  in 
either  experiment  ? 

Suppose  that  the  needle  of  a  compass  had  lost  its  magnet- 
ism, how  would  you  restore  it  and  have  the  polarity  the  same 
as  before  ? 

Suggestions.  —  Test  all  iron  pipes  and  rods  about  the  labora- 
tory. Where  do  you  find  the  -f-  pole  ?  Where  the  —  pole  ? 
WThere  the  neutral  point  ?  Why  are  the  rods  magnetized  ? 


EXPERIMENT   34 

Study  of  a  Single-fluid   Galvanic   Cell.      El.  of   Phys., 

pp.  305-307 

Apparatus.  —  A  strip  of  sheet  zinc ;  a  similar  sheet  of  thin 
copper;  a  tumbler  two  thirds  full  of  dilute  sulphuric  acid 
(H2S04  1  part,  water  10  parts);  a  few  drops  of  mercury; 
compass ;  connecting  wires. 

Method.  — Stand  the  strip  of  zinc  in  the  sulphuric  acid  and 
observe  its  surface.  Do  the  same  with  the  copper  strip. 
Bring  the  upper  ends  of  the  strips  into  contact  outside  the 
liquid  and  observe  any  change  in  the  action  on  the  surface  of 
each. 

Remove  both  from  the  liquid  and  amalgamate  the  end  of  the 
zinc  strip  that  was  in  the  liquid  by  rubbing  mercury  over  it. 
Repeat  the  experiments  with  the  amalgamated  zinc  both  alone 
and  with  the  copper.  Solder  a  copper  wire  to  the  upper  end 
of  each  strip  and  connect  the  wires.  Turn  the  apparatus  so 
that  the  direction  of  the  wires  is  north  and  south.  Then  hold 
the  compass  under  each  wire  and  determine  the  direction  of 
the  current.  This  can  be  done  by  holding  the  right  hand  so 
that  the  wire  is  between  the  palm  of  the  hand  and  the  coin- 
pass  needle.  If  the  north  end  of  the  needle  is  deflected  toward 
the  extended  thumb,  the  direction  of  the  current  in  the  wire  is 
from  the  wrist  toward  the  finger  tips. 

Conclusion.  —  What  is  the  cause  of  the  "local  action"  on 
the  surface  of  the  zinc  before  it  has  been  amalgamated  ?  How 
does  amalgamating  the  zinc  prevent  this  action?  Which 
pole  of  the  cell  is  +  and  which  is  —  ? 


SINGLE-FLUID   CELLS 


EXPERIMENT   35 

The  E.M.F.  of  a  Single-fluid  Cell,  and  the  Electromotive 
Series.     El.  of  Phys.,  pp.  308,  312-314 

Apparatus.  —  Strips  of  aluminum,  carbon,  copper,  iron,  lead, 
and  zinc ;  a  glass  tumbler,  or  better  the  form  of  simple  cell 
shown  in  Fig.  41;  a  voltmeter;  dilute 
sulphuric  acid ;  dilute  hydrochloric  acid  j 
connecting  wires ;  switch. 

Method.  —  Make  a  cell  using  dilute 
sulphuric  acid,  and  the  copper  and  zinc 
strips.  Couple  the  voltmeter  to  the  cell 
and  take  a  reading.  Lift  the  strips 
nearly  out  of  the  liquid,  keeping  them 
at  the  same  distance  apart,  and  take  the 
reading.  Push  the  strips  nearer  together 
without  changing  their  depth  in  the 
liquid,  and  read  the  voltmeter.  Take  a  FIG.  41. 

set  of  readings  using  the  carbon  strips 

with  all  the  others  in  succession ;  take  another  set  using  the 
copper  strip  with  all  the  others  in  succession,  and  so  on  until 
all  possible  combinations  have  been  made.  Wash  the  strips 
and  take  similar  sets  of  readings,  using  dilute  hydrochloric 
acid  as  the  liquid. 

Conclusions.  —  What  effect  does  the  extent  of  the  surface  of 
the  strips  in  the  liquid  have  upon  the  voltage  of  the  cell  ? 
What  effect  does  the  distance  of  the  strips  from  each  other 
have  upon  the  voltage  ?  Is  the  voltage  constant  for  some 
time  with  all  the  cells  ? 

Suggestion.  —  Make  a  list  of  the  substances  used,  arranging 
them  in  order,  placing  at  the  top  the  one  that  is  positive  to  all 
the  rest,  next  the  one  that  is  positive  to  all  except  the  first, 
and  so  on  to  the  last  in  the  list,  which  will  be  negative  to  all 


70  ELECTRICITY 

the  rest.  Make  a  list  for  each  liquid  used.  Do  the  substances 
come  in  the  same  order  in  the  two  lists  ?  What  two  substances 
give  the  greatest  difference  of  potential  in  each  ? 

EXPERIMENT   36 

To  study  a   Two-fluid   Cell  and  to  compare  with  it  a 
Dry  Cell.     El.  of  Phys.,  pp.  308-311 

Apparatus.  —  A  cell  like  that  used  in  Experiment  35 ;  a  strip 
of  copper  and  one  of  amalgamated  zinc ;  a  small  porous  cell ; 
dilute  sulphuric  acid  ;  solution  of  copper  sulphate  ;  a  dry  cell ; 
an  ammeter  ;  voltmeter ;  connecting  wires ;  switch. 

Method.  — Pour  the  copper  sulphate  solution  into  the  tumbler 
until  it  is  about  half  full.  Set  the  porous  cup  in  the  liquid 
and  pour  dilute  sulphuric  acid  into 
the  cup  until  the  two  liquids  stand 
at  the  same  height.  Insert  the  copper 
strip  in  the  copper  sulphate  solution 
and  the  zinc  strip  in  the  sulphuric 
acid.  Couple  the  voltmeter  to  the  cell 
and  read  the  voltage.  Couple  the 
ammeter  and  the  switch  in  series  to 
FIG.  42.  the  cell,  and  couple  the  voltmeter  as 

in  Fig.  42.     Close  the  switch  and  read 

the  ammeter  and  voltmeter.  Leave  the  switch  closed  for  five 
minutes.  Read  the  voltmeter  and  ammeter.  Open  the  switch 
and  read  the  voltmeter.  Close  the  switch  again  for  another 
five  minutes  and  repeat  the  readings.  Do  this  for  a  half  hour 
and  tabulate  your  results.  Repeat  the  experiments,  substituting 
a  dry  cell  for  the  two-fluid  cell,  and  tabulate  your  results. 

Conclusion. — The  two-fluid  cell  is  called  a  "closed  circuit 
cell."  What  evidence  does  the  experiment  give  showing  that 
it  is  adapted  to  this  purpose  ?  How  does  the  dry  cell  compare 
with  it  in  this  respect  ?  Would  the  dry  cell  answer  for  "  open 


ARRANGEMENT   OF   CELLS  71 

circuit"  work?     Why?     Write  a  description  of  the  results  of 
polarization  in  a  cell. 

Suggestions.  —  Couple  a  new  dry  cell  to  the  voltmeter  and 
make  a  record  of  its  voltage.  Couple  four  such  cells  in  series 
and  record  their  voltage.  Couple  the  same  four  cells  in 
parallel  and  record  their  voltage.  Write  the  results  of  your 
experiment  and  the  results  of  series  and  parallel  coupling. 

EXPERIMENT   37 

Arrangement  of  Cells  to  produce  the  Greatest  Current  in 
a  Given  Circuit.     El.  of  Phys.,  p.  316 

Apparatus.  —  Several  two-fluid  cells,  or  any  cells  that  will 
give  a  fairly  constant  potential ;  copper  wire  No.  18 ;  German 
silver  wire  No.  30 ;  a  galvanometer  with  both  high  and  low 
resistance  coils ;  connecting  wires ;  a  switch. 

Method.  —  Couple  the  cells  in  series  and  for  the  external 
resistance  put  10  ft.  of  No.  18  copper  wire,  the  low  resist- 
ance coil  of  the  galvanometer,  and  the  switch.  Close  the 
switch  and  read  the  deflection.  Couple  the  cells  in  parallel 
and  read  the  deflection.  Repeat  both  readings  after  repla- 
cing the  copper  wire  by  10  ft.  of  German  silver  wire  No.  30. 
Couple  the  high  resistance  coil  of  the  galvanometer  in  the  cir- 
cuit and  repeat  all  readings. 

Conclusion.  —  From  the  readings  taken  in  each  case  deter- 
mine which  is  the  better  arrangement  of  cells  for  a  low  ex- 
ternal resistance  and  which  for  a  high  external  resistance. 

EXPERIMENT   38 

Effect  of  an  Electric  Current  upon  the  Temperature  of 
a  Conductor.    El.  of  Phys.,  p.  317 

Apparatus.  —  Four  or  more  cells;  10  ft.  of  iron  or  German 
silver  wire  No.  30  ;  galvanometer ;  thermometer ;  connecting 
wire;  switch. 


72  ELECTRICITY 

Method.  —  Wind  a  part  of  the  wire  into  a  coil  closely  around 
the  bulb  of  the  thermometer,  leaving  the  rest  of  the  wire  as  a 
resistance.  If  the  wire  is  not  insulated,  the  turns  of  the  coil 
must  be  pulled  apart  so  that  they  do  not  touch  one  another. 

Couple  the  coil,  cells,  galvanometer,  and  switch  in  series  and 
send  a  current  through  the  circuit.  Take  such  a  number  of 
cells  as  will  give  a  suitable  deflection  of  the  galvanometer,  and 
such  a  galvanometer  that  a  large  current  will  be  required  to 
deflect  it.  Take  simultaneous  readings  of  the  galvanometer 
and  thermometer.  Reduce  the  length  of  wire  to  about  half 
and  again  send  a  current  through  the  circuit  and  take  readings. 

Conclusion.  —  Does  the  passage  of  a  current  in  a  wire  change 
its  temperature  ?  What  is  the  change  ?  Does  the  extent  of 
the  change  depend  upon  the  amount  of  the  current  ? 


EXPERIMENT   39 

Lines  of  Force  about  a  Current-carrying  Conductor. 

El.  of  Phys.,  p.  318 

Apparatus.  —  A  battery  of  six  cells ;  a  smooth  card  and  sup- 
port ;  insulated  wire  No.  18  ;  six  small  compasses  one  half 
inch  or  less  in  diameter  ;  switch. 

Method.  —  Support  the  card  horizontally  and  run  the  insu- 
lated wire  vertically  through  it,  connecting  it  in  series  with 

the  cells  and  switch.  Place  the 
compasses  on  the  card  in  a  circle  as 
close  to  the  wire  as  possible. 

Conclusion.  —  Observe  the  direc- 
tion taken  by  the  compass  needles 
both  before  and  after  the  current  is 
turned  on.  Make  drawings  of  the 
same.  Reverse  the  direction  of  the 
current  in  the  conductor  by  changing 
FIG.  43.  the  couplings,  and  observe.  Write 


ELECTRO-MAGNET  73 

the  law  of  the  relation  of  the  current  in  the  conductor,  and  the 
direction  of  the  magnetic  field  around  it. 

Suggestion.  —  If  a  current  of  20  to  30  amperes  is  at  hand, 
a  very  good  result  can  be  obtained  with  iron  filings  sifted 
on  the  card  around  the  wire.  A  good  effect  can  be  obtained 
with  a  battery  current  if  the  wire  is  wound  into  a  coil,  both 
branches  of  which  pierce  the  card,  as  shown  in  Fig.  43. 


EXPERIMENT   40 
To  study  an  Electro-magnet.     El.  of  Phys.,  p.  321 

Apparatus.  —  A  rod  of  soft  iron  4  in.  long  and  a  quarter  .of 
an  inch  in  diameter;  insulated  copper  magnet  wire  No.  18; 
two  cells  ;  iron  filings  ;  wire  nails  ;  compass ;  connecting  wires ; 
galvanometer ;  switch. 

Method.  —  Wind  the  wire  around  the  iron  rod  until  nearly 
its  whole  length  is  covered,  and  fasten  the  ends  of  the  coil  to 
the  rod  with  thread.  Test  the  ends  of  the  rod  with  the  com- 
pass needle  for  magnetism.  Couple  a  cell,  the  coil,  galvanom- 
eter, and  switch  in  series  and  close  the  switch.  Test  the  two 
ends  of  the  rod  for  magnetism. 

Reverse  the  current  in  the  coil  and  again  test  the  rod. 
While  the  current  is  on,  dip  the  end  of  the  rod  in  a  pile  of 
small  nails.  Break  the  circuit  and  observe  the  action  of  the 
nails.  Couple  two  cells  in  the  circuit  in  the  way  that  will  give 
the  maximum  current,  and  while  the  current  is  on,  insert  the 
end  of  the  rod  in  the  nails. 

Fix  the  rod  in  a  vertical  position  and  sift  iron  filings  on  a 
card  supported  horizontally  on  the  upper  end,  and  observe  the 
lines  of  force. 

Conclusion.  —  Make  a  drawing  showing  the  relation  between 
the  direction  in  which  the  current  passes  about  the  rod  and  the 
polarity  of  its  ends.  What  effect  does  the  amount  of  current 
in  the  coil  have  upon  the  number  of  nails  the  rod  will  carry  ? 


74  ELECTRICITY 

Suggestion.  — Wind  a  solenoid  in  the  form  of  Fig.  44.     Send 
a  current  through .  it  from  a  cell  and  observe  the  effect  it  has 

upon  a  compass  needle  placed 
at  A,  B,  C,  and  D  in  the 
figure.  Mark  in  your  notes 
the  direction  of  the  current 

passing  from  the  cell  to  the 

FIG>  44.  solenoid  and  its  direction  in 

the  coil.     Without  changing 

the  position  of  either  the  solenoid  or  the  compass,  introduce 
a  piece  of  gas  pipe  or  any  other  iron  core  into  the  solenoid 
and  explain  the  result. 


EXPERIMENT  41 

To  make  and  study   a   Lifting  Magnet.      El.  of   Phys., 

p.  322 

NOTE.  —  It  may  not  be  possible  for  all  schools  to  make  this  experiment. 
Those  schools  which  can  make  it,  or  some  modified  form  of  it,  will  find  it 
most  instructive  and  interesting. 

Apparatus.  — Lighting  circuit  as  source  of  current ;  ammeter ; 
lamp  resistance  board;  special  apparatus  described  below. 

Method.  —  In  order  that  an  electro-magnet  may  have  great 
lifting  power  it  should  have  a  small  air  gap  and  a  short  iron 
circuit,  and  the  iron  should  be  of  high  permeability ;  that  is,  it 
should  offer  little  resistance  to  the  lines  of  force. 

An  efficient  form  is  shown  in  Fig.  45,  a  part  of  which  is 
in  section.  I  is  the  iron  of  the  magnet  and  I'  that  of  the 
armature.  This  is  known  as  an  iron-clad  form,  and  fulfils  the 
first  and  second  of  the  required  conditions.  The  coil  (7  is 
wound  upon  a  former,  is  wrapped  with  tape,  and  fitted  into  the 
groove  in  the  iron.  Wis  a  wooden  ring  that  serves  to  support 
the  binding  posts  PP'. 

To  make  this  magnet,  turn  on  a  wood  lathe  patterns  for  the 


LIFTING  MAGNET 


75 


FIG.  45. 


magnet     and     armature.1 

Have    castings    made    of 

iron  or  soft  steel  and  file 

or  turn  the  contact  faces 

to  make  a  good  fit.     Turn 

a   wooden    spool    with   a 

groove      in      it      slightly 

smaller  than  the  size  of 

the  coil,  wind   it  full  of 

magnet  wire  No.  24,  then 

wind  this  coil  with  tape 

and    give    it    a    coat    of 

shellac.      It    is    a    good 

plan  to  make  a  split  spool 

as  shown   in   Fig.  46,  so 

that  it  will  be  possible  to 

take  out  the  screws  that  hold  the  halves  together,  and  remove 

the  coil  without  pulling  it  apart. 
Force  the  coil  into  the  groove 
of  the  electro-magnet  before  the 
shellac  is  dry,  and  couple  the 
wires  to  the  binding  posts  as 

shown.     Two  holes  must  be  drilled  through  the  iron,  through 

which  the  ends  of  the  coil  may  be  connected  with  P  and  P'. 

The  dimensions  may  well  be  as  follows :  — 

Diameter  of  magnet  four  and  one  half  inches. 
Outside  diameter  of  coil  three  and  one  half  inches. 
Inside  diameter  of  coil  two  and  one  half  inches. 
Depth  of  coil  one  half  inch. 
Thickness  of  armature  one  half  inch. 

The  iron  parts  can  be  obtained  from  any  machine  shop  if  pre- 
ferred. 


Y///////, 


FIG.  46. 


1  If  there  is  a  heavy  iron  casting  with  a  flat  face  at  hand,  it  can  be  used  in 
place  of  the  armature. 


76 


ELECTRICITY 


In  order  to  make  a  study  of  the  lifting  power  it  will  be 
best  to  make  the  special  piece  of  apparatus  shown  in  Fig.  47. 
Get  a  wooden  bar  about  8  ft.  long  and  2  in.  or  more  square, 
and  at  one  end,  as  A,  put  in  a  heavy  screw  hook.  Weigh  the 
bar  and  determine  its  center  of  gravity.  Bore  a  half-inch  hole 
at  a  distance  of  1  ft.  from  A  and  drive  a  half-inch  iron  rod 
through  the  bar  for  a  fulcrum.  Put  a  large  ring  over  the  bar 
and  from  it  suspend  a  weight,  20  to  50  Ib.  Weigh  the  ring  and 
add  its  weight  to  that  of  the  weight  used.  Suspend  the  magnet 
from  the  hook  at  A  and  fasten  the  armature  to  a  screw  eye  in 


FIG.  47. 

the  floor.  Couple  the  magnet  coil  to  a  lighting  circuit  with  a 
lamp  board  and  ammeter  in  series  with  it.  Send  the  current 
through  one  lamp,  slide  the  weight  slowly  along  the  bar  away 
from  the  fulcrum,  and  record  the  position  of  the  weight  when 
the  magnet  is  separated  from  the  armature.  It  is  well  to  put 
a  box  under  the  end  of  the  bar  at  B  to  prevent  its  falling  to 
the  floor  when  the  armature  is  pulled  off.  Take  a  reading  of 
the  current  at  the  exact  time  of  separation.  Add  another 
lamp  and  again  find  the  position  of  the  weight  and  the 
amount  of  the  current.  Keep  adding  lamps  and  taking  read- 
ings until  the  current  has  reached  3  amperes.  Too  great  a 
current  will  overheat  the  coil  of  the  electro-magnet  and  may 
break  the  insulation. 

Conclusion.  —  Make  a  curve  showing  the  relation  of  the  cur- 
rent passing  in  the  coil  of  the  electro-magnet  to  the  lifting 
power  of  the  magnet. 


RESISTANCE   OE  A   CONDUCTOR  77 

EXPERIMENT   42 
Electroplating.     El.  of  Phys.,  p.  325 

Apparatus. — Cell;  connecting  wires  ;  copper  sulphate  solu- 
tion ;  large  steel  wire  nails ;  glass  beaker. 

Method.  —  Scour  the  nails  thoroughly  with  sandpaper  and 
wash  them  off  with  clean  water.  Wind  around  the  head  of 
each  a  half  dozen  turns  of  copper  wire  from  which  the  insula- 
tion has  been  stripped.  Couple  two  or  three  cells  in  series 
and  connect  one  nail  with  the  -f  and  the  other  with  the.  - 
pole  of  the  battery.  Dip  the  nails  into  the  solution  of  copper 
sulphate  and  observe  what  takes  place  on  each  nail.  After 
the  battery  has  been  sending  current  for  about  ten  minutes 
take  out  the  nails  and  examine  them.  Wash  and  dry  them 
and  try  the  effect  of  rubbing  each  of  them  with  an  eraser. 

Change  the  connections  of  the  nails  to  the  battery  so  that 
the  one  that  was  connected  to  the  +  will  now  be  connected  to 
the  —  pole  and  the  one  that  was  connected  to  the  —  will  now 
be  connected  to  the  -f  pole.  Dip  the  nails  again  into  the  solu- 
tion and  observe  what  takes  place. 

Conclusion.  —  What  was  deposited  on  the  cathode,  i.e.  the 
nail  connected  with  the  —  pole  of  the  battery  ?  What  was 
set  free  at  the  anode,  the  nail  connected  with  the  +  pole? 
Does  the  copper  ion  in  the  copper  sulphate  solution  go  in  the 
same  direction  as  the  current  in  the  solution  or  in  the  opposite 
direction?  Answer  the  same  question  about  the  ions  of  oxygen. 
With  what  kind  of  electricity  are  the  copper  ions  charged,  + 
or  -? 

EXPERIMENT   43 

To  study  the  Resistance  of  a  Conductor.     El.  of  Phys., 

p.  329 

Apparatus.  — A  two-fluid  cell ;  insulated  copper  magnet  wire 
Nos.  27  and  30 ;  German  silver  wire  No.  30 ;  a  galvanometer ; 
connecting  wires ;  switch. 


78  ELECTRICITY 

Method.  —  Put  5  ft.  of  copper  wire  No.  30  in  series  with 
the  cell,  galvanometer,  and  switch.  Throw  on  the  current  and 
read  the  deflection.  Couple  in  10  ft.  of  the  same  wire  and 
read  the  deflection.  Couple  in  a  double  wire  made  of  two 
wires  like  the  last  and  read  the  deflection.  Couple  in  10  ft. 
of  wire  No.  27  and  read  the  deflection.  Couple  in  10  ft.  of 
German  silver  wire  No.  30  and  read  the  deflection. 

Conclusion.  —  Compare  the  deflection  obtained  with  the 
5  ft.  of  copper  wire  and  that  obtained  with  the  10  ft.  Com- 
pare the  deflections  with  a  single  wire  and  with  a  double  wire. 
Compare  the  deflection  with  the  double  wire  No.  30  and  that 
with  the  single  wire  No.  27.  Compare  the  deflection  with 
10  ft.  of  copper  wire  and  that  with  10  ft.  of  German  silver 
of  the  same  size. 

Do  your  readings  indicate  that  the  expression  for  the  resist- 
ance of  a  wire,  R  =  A"~,  is  correct  or  not  ?  In  this  expression, 

AT  represents  the  relative  resistance  of  different  materials,  I  the 
length,  and  d  the  diameter  of  the  wire.  Refer  to  the -wire 
table  for  the  diameter  of  wires  Nos.  27  and  30. 


EXPERIMENT   44 

Effect  of  Temperature  on  the  Resistance  of  a  Conductor. 
Study  of  the  Change  of  Resistance  of  an  Iron  Wire 
due  to  Change  of  Temperature.  El.  of  Phys.,  p.  329 

Apparatus.  — Small  iron  wire;  cells  ;  galvanometer  ;  a  wooden 
spool;  beaker  of  water;  Bunsen  burner;  a  quarter-inch  iron 
rod  ;  connecting  wires  ;  switch. 

Method.  —  Wind  the  iron  wire-  in  a  close  spiral  around  the 
iron  rod.  Carefully  remove  the  spiral  and  wind  it  around  the 
spool  so  that  the  turns  of  the  wire  shall  not  touch  one  another. 
Place  the  spool  with  its  coil  in  the  beaker  of  water,  submerging 
the  entire  coil.  Couple  it  in  series  with  the  cell,  galvanometer, 


FALL  OF  POTENTIAL   METHOD  79 

and  switch.  Send  the  current  through  the  circuit  and  take  a 
reading  of  the  galvanometer  deflection.  Heat  the  water  in  the 
beaker  nearly  to  the  boiling  point  and  repeat  the  experiment. 

Conclusion.  —  Compare  the  readings  of  the  galvanometer  in 
the  two  experiments  and  state  what  effect  increasing  the  tem- 
perature of  an  iron  wire  has  upon  its  electrical  resistance. 
How  does  the  hot  resistance  of  an  incandescent  lamp  compare 
with  its  cold  resistance  ?  Does  a  rise  of  its  temperature  in- 
crease or  diminish  the  resistance  of  carbon  ? 

Suggestion.  —  If  the  beaker  is  looked  upon  as  a  calorimeter, 
it  will  be  an  interesting  addition  to  the  experiment  to  deter- 
mine the  number  of  calories  given  to  the  water  by  the  elec- 
trical heating  of  the  iron  wire  (compare  Experiment  38),  and  to 
see  whether  it  verifies  the  formula  for  the  heating  effect  of  a 
current ;  namely,  Calories  =  0.24  C2Rt, 


EXPERIMENT   45 

To  measure  the  Resistance  of  a  Conductor  by  the  Fall  of 
Potential  Method.     El.  of  Phys.,  p.  337 

If  an  electric  current  passes  through  a  conductor,  there 
must  be  a  difference  of  potential  between  the  two  ends  of  the 
conductor.  This  is  called  the  fall  of  potential  through  the 
conductor.  If  this  fall  of  potential  and  the  current  that 
passes  through  the  conductor  are  known,  the  resistance  of  the 
conductor  can  be  found  by  substituting  these  values  in  the  ex- 

PD 

pression  for  Ohm's  Law,  from  which  R  —  -—. 

C 

Apparatus.  —  The  conductor,  the  resistance  of  which  is  to 
be  measured;  an  ammeter;  voltmeter;  several  constant  poten- 
tial cells,  or  a  lighting  circuit  as  source  of  current ;  switch ; 
rheostat ;  connecting  wires. 

Method.  —  Couple  the  conductor,  rheostat,  ammeter,  and 
switch  in  series  with  the  source  of  current,  as  in  Fig.  48. 


80 


ELECTRICITY 


-  48- 


Couple  the  voltmeter  as  a  shunt  to  the  conductor.  Put  in  all 
the  -resistance  of  tho  rheostat.  Close  the  switch  and  read  the 
ammeter  and  voltmeter  at  the  same 
instant.  This  can  be  done  by  having 
one  student  give  a  signal  to  two  others 
who  do  the  reading.  Reduce  the  resist- 
ance of  the  rheostat  and  take  a  second 
set  of  readings.  Take  a  number  of 
sets  of  readings,  reducing  the  rheostat 
resistance  each  time  until  the  current 
is  as  great  as  you  wish  to  use. 
Conclusion.  —  Compute  the  resistance  for  each  set  of  read- 
ings. Are  the  resistances  the  same  ?  Would  the  change  in 
temperature  explain  the  differences  ?  With  a  low-reading  volt- 
meter and  a  large  current  this  is  a  convenient  and  accurate 
method  for  the  measurement  of  small  resistances. 

Suggestions.  —  It  is  often  convenient  to  be  able  to  set  the 
arm  of  a  rheostat  at  a  known  resistance.  In  order  to  have  a 
permanent  record  for  this  purpose,  measure  the  resistance  of 
the  rheostat  by  this  method  and  make  a  curve  from  the 
results  of  your  measurement  that  shall  show  the  relation 
between  the  resistance  of 
the  rheostat  and  the  dif- 
ferent contact  points. 

Make    a    study    of    the 
parallel   resistance   of    in- 
candescent lamps  by  put- 
ting a  number  of  110-volt  lamps  of  different  candle  power  in 
a  lamp  board,  as  in  Fig.  49.     Find   the   resistance  of   each 
lamp   and   then   the   resistance   of   2,  3,  4,  etc.,  in   parallel. 
Compare  the  resistance  with  that  computed  from  the  formula, 


FIG.  49. 


1' 


etc. 


DIVIDED   CIRCUIT 


81 


I 


EXPEEIMENT   46 

The  Distribution  of  Current   over  the  Branches   of   a 
Divided  Circuit.     EL  of  Phys.,  p.  338 

A  divided  circuit  is  one  that  provides  two  or  more  paths 
between  two  points.  A  study  of  the  currents  in  the  two 
branches  of  the  circuit  (called  also  shunt  or  parallel  circuits) 
may  be  made  as  follows. 

Apparatus. — Three  ammeters;  three  rheostats;  a  number 
of  cells  or  a  dynamo  circuit ;  connecting  wires. 

Method.  —  Couple  the  apparatus  as  in  Fig.  50.  It  is  evi- 
dent that  ammeter  A  will  read  the  entire  current  and  that 
its  reading  will  be  the  sum 
of  the  readings  of  A'  and  r~ 
A".  Make  a  number  of 
sets  of  readings,  varying 
the  resistance  of  R  to  con- 
trol the  main  current  and 
that  of  R1  and  E"  to  con- 
trol the  current  in  the 
branches. 

Conclusion.  —  The     re- 
sults of  your  experiments 
should  show  clearly  the  effect  of  changing  the  parallel  resist- 
ances in  a  circuit  upon  the  currents  in  the  two  paths. 

Suggestions.  —  If  the  resistances  of  one  of  the  rheostats,  as 
R\  are  laid  off  on  the  horizontal  axis  and  the  currents  in  A'  on 
the  vertical  axis,  the  resulting  curve  will  be  a  good  example 
of  an  inverse  change,  i.e.  one  in  which  one  value  decreases  and 
the  dependent  value  increases. 

If  the  three  ammeters  and  rheostats  are  not  at  hand,  use 
two  galvanometers  —  one  in  each  branch  —  and  change  the 
currents  by  putting  in  copper  and  German  silver  wires  and 
interchanging  them. 

PHYS.   LAB.  BOOK 6 


FIG.  50. 


Q   Aft 


82  ELECTRICITY 


EXPERIMENT   47 

To  measure  the  Resistance  of  a  Conductor  by  the  Wheat- 
stone  Bridge.     El.  of  Phys.,  p.  338 

Apparatus.  —  A  slide- wire  bridge  ;  constant  voltage  cell ; 
resistance  box;  a  sensitive  galvanometer;  four  pieces  of  wire 
No.  30,  each  10  ft.  long,  of  copper,  iron,  German  silver,  and 
climax  wire ;  connecting  wires ;  switch. 

Method.  —  Couple  the  apparatus  as  shown  in  Fig.  51,  putting 
in  the  copper  wire  at  Xas  the  one  to  be  measured  first.  Make  an 

estimate  of  the  resistance  of  the 

^|l| — 0 ,^     wire  and  arrange  the  plugs  in  the 

resistance  box  to  read  that  amount. 
Close  the  switch  and  touch  down 
the  key  K  at  the  middle  of  the 
wire  AB.  Observe  the  direction 
and  amount  of  the  deflection  of  the 

-^~X  JL* 

FlG  51>  galvanolneter     needle.       Increase 

the  resistance  of  the  box  and  touch 

down  the  key  again.  Judge  by  the  direction  and  amount  of  the 
deflection  whether  the  resistance  of  the  box  is  too  little  or 
too  much.  Change  the  resistance  of  the  box  as  required  until 
there  is  no  deflection  of  the  galvanometer  needle.  If  the 
resistances  in  the  box  are  not  small  enough  to  give  this  re- 
sult, it  can  be  brought  about  by  moving  the  key  K  towards  A 
or  B.  When  this  position  is  found  for  /i,  the  proportion 
X:R  =  AK:  KB  holds,  and  hence  X  X  KB  =  R  x  AK,  that  is, 
the  cross  multiplications  of  the  bridge  arms  are  equal.  If  7T  is 
at  the  middle  of  AB,  it  is  evident  that  X  =  R.  Find  the  value 
of  X.  In  the  same  way  find  the  resistance  of  all  the  wires. 

Conclusion.  —  Since  the  wires  are  of  the  same  length  and 
diameter,  the  reflation  of  their  resistances  will  be  the  relation 
of  their  specific  resistances  and  should  be  the  same  as  that 
given  in  a  table  of  specific  resistance. 


THE   INTERNAL   RESISTANCE   OF   A   CELL  83 

A  very  convenient  form  of  the  Wheatstone  bridge  is  the 
portable  testing  set.  This  has  the  cells,  galvanometer,  re- 
sistance coils,  and  a  form  of  ping  resistance  box  to  take  the 
place  of  the  slide  wire,  all  in  a  small  box  that  can  be  conven- 
iently carried  about. 

Pupils  cannot  become  too  familar  with  the  different  forms 
of  the  Wheatstone  bridge,  since  it  is  applied  in  very  many 
ways  in  practical  work. 


EXPERIMENT   48 
The  Internal  Resistance  of  a  Cell.     El.  of  Phys.,  p.  341 

(a)  To  study  the  internal  resistance  of  a  cell. 

Apparatus. — A  simple  cell;  low-resistance  galvanometer, 
or  ammeter;  connecting  wires;  switch. 

Method.  —  Couple  the  cell,  galvanometer,  and  switch  in 
series.  Take  a  reading  of  the  deflection  of  the  galvanometer 
with  the  zinc  and  copper  strips  immersed  to  nearly  the  full 
length.  Keeping  the  strips  at  exactly  the  same  distance  from 
each  other,  pull  them  np  and  clamp  them  in  such  a  position 
that  they  are  only  immersed  two  thirds  their  length.  Take  a 
second  reading  of  the  deflection.  Repeat  with  the  strips  im- 
mersed to  but  one  third  their  length.  Again  clamp  the  strips 
so  that  their  full  length  is  immersed  and  separate  them  as  far 
as  possible,  keeping  them  parallel  to  each  other.  Take  a  read- 
ing of  the  deflection.  Eepeat  several  times,  reducing  the 
distance  between  the  plates  each  time  until  they  are  nearly 
touching. 

Conclusion.  —  From  a  comparison  of  the  records  that  you 
have  taken,  what  is  the  effect  upon  the  current  of  changing 
the  area  of  the  submerged  part  of  the  strips?  Of  changing 
the  distance  between  the  strips?  Since  the  only  change  was 
in  either  the  distance  apart  of  the  copper  and  zinc  plates  in 
the  cell  or  the  amount  of  the  plate  that  was  inserted  in  the 


'84  ELECTRICITY 

liquid,  any  change  in  the  current  would,  from  Ohm's  Law, 
(7  =  — ,  indicate  an  opposite  change  in  the  resistance  of  the 

cell.  What  changes  in  internal  resistance  does  the  experiment 
indicate  ?  Does  the  internal  resistance  of  a  cell  follow  the 
law  of  resistance  of  a  conductor  ?  How  do  your  results  show 
this? 

(6)  To  measure  the  internal  resistance  of  .a  cell ;  the 
half-deflection  method. 

Since  a  cell  is  a  generator  of  the  electric  current  and  polari- 
zation is  apt  to  take  place  in  it,  the  measurement  of  its  resist- 
ance offers  more  difficulties  than  does  the  measurement  of  an 
ordinary  resistance. 

Apparatus.  —  The  cells  the  resistance  of  which  is  to  be 
measured;  an  ammeter  that  will  give  an  accurate  reading  for 
small  currents;  rheostat;  a  testing  set  or  Wheatstone  bridge; 
connecting  wires ;  switch. 

Method. — Connect  the  rheostat,  ammeter,  switch,  and  the 
wire  necessary  to  couple  in  the  cell,  and  measure  the  resistance 
with  the  testing  set  when  there  is  no  resistance  in  the  rheostat. 
Couple  in  the  cell  with  the  switch  open.  Close  the  switch  and 
read  the  ammeter.  This  reading  should  be  taken  at  the  instant 
the  needle  comes  back  from  its  first  throw,  and  before  it  begins 
to  fall  slowly.  This  slow  fall  will  take  place  with  all  cells 
that  polarize.  Introduce  resistance  into  the  circuit  by  the 
rheostat  until  the  ammeter  reads  just  one  half  the  former 
reading.  Be  careful  to  open  the  switch  when  the  current  is 
not  needed. 

Conclusion.  —  Call  the  first  resistance  measured  by  the  test- 
ing set  r,  the  resistance  added  by  the  rheostat  R,  the  resistance 
of  the  cell  x,  and  its  electromotive  force  E.  Then  the  first 

Tjl 

current  will  be  givei?  by  the  expression  O  =—  — ;  the  second 

r  -f-a? 


CUTTING   LINES   OF   FORCE    WITH   CONDUCTOR          85 


current   will   be    C"  =  —  -  r-    Since    <7  =  2O",  and  E  is 


F  2  F 

the  same  in  both  expressions.  -  =  -  ^,  from  which  we 

r+x     r+x+R 

get  x=  R  —  r.     Apply  the  method  to  a  number  of  different 
cells  and  compute  the  resistance  of  each. 

(c)  The  voltmeter  method. 

Apparatus.  —  A    low-reading   voltmeter  ;    an    ammeter  for 
small  currents  ;  the  cells  the  resistance  of  which 
is  to  be  tested  ;  connecting  wires  ;  switch. 

Method.  —  Couple  the  apparatus  as  in  Fig.  52. 
Take  first  the  reading  of  the  voltmeter  when  the 
switch  in  the  ammeter  circuit  is  open.  Call  this 
reading  V.  Close  the  switch  and  take  simulta- 
neous readings  of  both  the  voltmeter  and  the 
ammeter.  Call  the  voltmeter  reading  V"  and 
the  ammeter  reading  C.  FlG>  52> 

Conclusion.  —  Compute  the  resistance  of  the  cell  from  the 

expression  R  =  ^  ~  V  -   Why?     This  method  is  well  adapted 
C 

to  the  measurement  of  the  resistance  of  a  cell  that  has  a  small 
resistance,  as  the  storage  cell,  for  example. 


-  EXPERIMENT  49 

Effect  of  Cutting  Lines  of  Force  with  a  Conductor. 
El.  of  Phys.,  pp.  344^347 

Whenever  a  conductor  passes  through  a  magnetic  field  in 
which  the  lines  of  force  are  not  parallel  to  the  conductor,  or 
whenever  a  conductor  "  cuts  lines  of  magnetic  force,"  an  elec- 
tric current  is  set  up  in  this  conductor  if  it  is  a  part  of  a 
closed  circuit. 


86  ELECTRICITY 

Apparatus.  —  A  bar  magnet ;  a  coil,  called  the  secondary, 
consisting  of  500  turns  of  No.  24  magnet  wire  wound  upon  a 
wooden  spool  about  half  as  long  as  the  magnet;  a  primary  coil 
of  50  turns  of  No.  18  magnet  wire  wound  upon  a  vulcanite  or 
pasteboard  tube,  the  coil  when  finished  being  of  such  a  size 
that  it  will  slip  easily  inside  the  secondary  coil ;  a  bundle  of 
iron  wires  as  core  for  the  primary ;  a  snap  switch ;  a  sensitive 
galvanometer,  or  milli  voltmeter;  a  rheostat;  cells;  connecting 
wires.  (See  Suggestions  below.) 

(a)  With  the  field  of  a  bar  magnet. 

Method.  —  First  determine  the  direction  of  current  in  the 
galvanometer  that  will  produce  a  throw  of  the  needle  to  the 
right. 

This  will  require  a  very  small  current  that  may  be  obtained 
by  connecting  the  galvanometer  as  a  shunt  to  a  short  piece  of 
wire  in  series  with  the  cells  and  rheostat. 

Connect  the  galvanometer  in  series  with  the  secondary  coil 
and  thrust  the  -f-  end  of  the  magnet  into  the  coil.  Observe 
the  direction  of  throw  of  the  needle.  Pull  out  the  magnet 
quickly  and  observe  the  direction  of  the  throw.  Eepeat, 
using  the  —  end  of  the  magnet. 

Conclusions.  —  Suppose  you  stand  in  the  magnetic  field 
facing  the  direction  toward  which  the  lines  of  force  are 
going.  Suppose  that  you  hold  a  conductor  in  your  hands 
horizontally,  and  then  allow  it  to  drop  so  that  its  motion  is 
vertically  downward,  what  will  be  the  direction  of  the  in- 
duced current  ? 

Make  a  drawing  to  illustrate  these  directions. 

Does  it  make  any  difference  whether  the  magnet  or  the  coil 
is  in  motion,  in  producing  the  cutting  of  the  lines  of  force  ? 

(b)  With  the  field  of  a  primary  coil. 

Method.  —  Couple  the  primary  coil  in  series  with  the  cells, 
rheostat,  and  snap  switch.  Close  the  switch  with  no  resist- 


CUTTING  LINES   OF   FORCE   WITH   CONDUCTOR 


87 


ance  in  the  rheostat  and  make  the  same  experiments  with  the 
primary  coil  as  with  the  bar  magnet  in  (a),  both  when  the  iron 
core  is  in  the  primary  and  when  it  is  not. 

Turn  off  the  current  and  place  the  primary  and  its  core 
inside  the  secondary.  Snap  the  switch  on  and  observe  the 
direction  and  magnitude  of  the  throw  of  the  needle.  Wait 
until  the  needle  is  quiet  and  then  snap  the  switch  off.  Ob- 
serve the  direction  and  magnitude  of  the  throw.  Introduce 
resistance  into  the  circuit  with  the  rheostat,  reducing  the 
current  in  the  primary,  and  repeat. 

Conclusion.  —  Make  a  drawing  to  show  the  relative  direction 
of  the  inducing  current  in  the  primary  coil,  and  of  the  induced 
current  in  the  secondary  coil,  both  on  starting  and  on  stopping 
the  current  in  the  primary. 

What  is  the  duration  of  the  induced  current  ? 

The1  cutting  of  the  lines  of  force  induces  an  electro-motive 
force.  If  the  cutting  conductor  is  part  of  a  closed  circuit,  there 
will  be  a  resulting  current;  its  magnitude  being  proportional 
to  the  induced  E.M.F. 

What  determines  the  magnitude  of  the  induced  E.M.F.  ? 

Suggestions.  —  Very  simple  apparatus  will  serve  for  experi- 
ment (a),  if  necessary.  A  coil  of  50  turns  of  copper  wire 
No.  30,  wound  on  a  cardboard  ring  of  such 
a  size  that  it  will  slip  freely  over  a  bar 
magnet,  serves  for  the  secondary  coil.  A 
simple  galvanometer  can  be  made  as  follows: 
Fasten  a  horseshoe  magnet  to  a  board,  as  in 
Fig.  53.  Wind  an  oblong  coil  of  fine  wire, 
No.  30,  for  example,  and  suspend  it  in  such 
a  way  that  it  will  hang  between  the  poles 
of  the  horseshoe.  A  slight  current  sent 
through  this  coil  will  give  it  a  throw  that  FlG 

will    determine   the   direction,   at    least,   of 
the  current,  if  the  direction  of  the  winding  of  the  coil   is 
known. 


ELECTRICITY 


EXPERIMENT   50 
To  study  a  Dynamo.     El.  of  Phys.,  pp.  352-359 

In  the  dynamo  the  conductors  in  which  the  E.M.F.  is 
induced  are  the  armature  wires  a;  the  lines  of  force  are  in 
the  air  gap  g,  between  the  poles,  N  and  S,  of  the  field  magnet 

/,  and  the  core  of  the  arma- 
ture c.  If  the  armature 
rotates  in  a  clockwise  direc- 
tion, the  lines  of  force  are 
cut  in  such  a  way  as  to  send 
the  current  through  the 
armature  coils  in  the  direc- 
tion indicated  in  Fig.  54. 

The  current  thus  gener- 
ated is  taken  off  by  the 
brushes  6,  as  they  come  in 
contact  with  the  commutator 
bars,  and  then  goes  through 
the  external  circuit  e.  In  the  figure  shown  a  part  of  the 
current  goes  around  the  field  magnet  coil  /,  and  produces  the 
magnetic  field. 

Apparatus. — A  small  shunt-wound  dynamo;  voltmeter; 
ammeter;  rheostats;  speed  counter;  a  small  compass;  con- 
necting wires;  switch. 

Method. — Examine  the  core  of  the  field  magnet  for  polarity 
with  the  compass  before  the  machine  is  started.  Kun  the 
armature  at  the  normal  speed,  which  may  be  determined  by 
the  use  of  the  speed  counter,  and  again  ex- 
amine for  polarity.  Couple  the  terminals 
of  the  machine  to  the  ammeter  and  one  of 
the  rheostats  in  series,  and  the  voltmeter 
in  parallel  (Fig.  55).  Introduce  a  resistance 
into  the  circuit  by  the  rheostat  and  run 


FIG.  54. 


FIG.  55. 


TO   STUDY  AN   ELECTRIC   MOTOR  89 

the  armature  at  the  normal  speed.  Take  readings  of  the 
ammeter,  voltmeter,  speed  counter,  and  resistance.  Reduce  the 
resistance  and  repeat,  running  the  armature  at  the  same  speed. 
Run  the  armature  at  different  speeds,  keeping  the  resistance 
the  same  and  taking  readings  of  the  ammeter  and  voltmeter. 
Introduce  resistance  into  the  shunt  field  circuit  by  the  use  of 
a  rheostat  and  run  the  armature  at  normal  speed  with  the 
same  resistance  in  the  main  circuit  as  at  first.  Bead  the  volt- 
meter. Disconnect  the  shunt  field  entirely  from  the  brushes, 
run  the  armature  at  normal  speed,  and  read  the  voltmeter. 

Whenever  two  or  more  instruments  are  read  in  this  experi- 
ment, the  reading  should  be  done  at  the  same  time. 

Conclusion. — Tabulate  your  results  and  answer  the  following 
questions :  — 

What  effect  upon  the  voltage  of  a  dynamo  does  speed  have? 
Why? 

What  effect  upon  the  current  does  resistance  in  the  main 
circuit  have?  Why? 

What  effect  upon  the  voltage  does  resistance  in  the  field 
coils  have?  WThy? 

Why  does  the  voltage  of  a  dynamo  increase  as  the  speed 
rises  from  zero  to  the  normal  rate? 

When  you  disconnect  the  shunt  field  circuit  from  the  brushes 
and  rotate  the  armature,  why  do  you  get  any  reading  with  the 
voltmeter  ? 

EXPERIMENT   51 
To  study  an  Electric  Motor.     El.  of  Phys.,  pp.  359-361 

The  cause  of  the  rotation  of  an  electric  motor  can  be  ex- 
plained by  considering  the  mutual  action  between  the  magnetic 
lines  of  force  in  the  air  gap  generated  by  the  current  in  the 
field  coils  and  that  generated  in  the  same  space  by  the  current 
in  the  armature  wires.  Figure  56  represents  the  lines  of  .force 
of  the  field  passing  across  the  air  gap  from  the  field  pole  to 


90 


ELECTRICITY 


the  armature  core,  and  the  circular  lines  of  force  around  the 
wire  due  to  the  current  in  the  wire. 

The  action  of  these  two  magnetic  fields  upon  each  other 
causes  the  wire  to  be  pushed  down  from  above  and  pulled  in 
the  same  direction  from  below.  . 

Since  the  armature  wires  are  fastened  to  the  circumference 
of  the  core  and  since  a  similar  action  is  taking  place  on  every 

armature  wire,  the  result  is  a  rotation  of 

the  armature. 

Apparatus. — A  small  shunt  motor  that 

can  be  run  by  a  battery  of  a  few  cells, 

or  a  110-volt  motor  on  a  lighting  circuit; 

ammeter;    voltmeter;    three   rheostats,   R, 

H',R"',  connecting  wires;   speed  counter; 

switch. 

Method. — Examine  the  connections  and  wiring  of  the  motor. 
Insert  one  rheostat,  R,  in  the  armature  circuit,  and  another,  11', 
in  the  shunt  field  circuit.  Couple  the  voltmeter  as  a  shunt 
to  the  terminals  and  put  an  ammeter  in  series  with  the 
feeding  wires.  Each  rheostat  shown  in  Fig.  57,  which  shows 


FIG.  57. 

the  connections,  has  an  extra  point  which  can  be  used  as  a 
switch.  Hold  the  armature  still  and  close  the  circuits  with 
resistance  in  R"  and  none  in  R  and  R'.  Take  readings  of 


TO   STUDY   AN   ELECTRIC    MOTOR  91 

both  the  ammeter  and  the  voltmeter  and  compute  the  parallel 
resistance  of  the  armature  and  the  shunt  field  coil.  Make 
the  proper  connections  and  take  readings  from  which  the 
resistances  of  the  armature  and  of  the  shunt  field  coil  can 
be  computed.  Send  the  current  through  the  shunt  field  coil, 
cutting  out  all  the  resistance  in  rheostat  R',  then  send  the 
current  through  the  armature,  leaving  all  the  resistance  in 
rheostat  R.  Reduce  the  resistance  of  R  until  the  armature 
begins  to  turn.  When  it  has  come  to  a  constant  speed, 
take  simultaneous  readings  of  the  ammeter,  voltmeter,  and 
speed  counter.  Reduce  the  resistance  of  R  still  more,  and 
when  the  armature  has  again  come  to  a  constant  speed, 
take  another  set  of  readings.  Change  the  resistance  of  the 
rheostat  until  the  motor  is  running  at  its  normal  speed  and 
again  take  a  set  of  readings. 

Conclusions.  —  Why  does  the  voltmeter  read  higher  when 
the  motor  is  running  at  its  full  speed  than  when  the  speed 
is  low  ? 

What  is  the  effect  of  an  increase  of  speed  upon  the  ammeter 
readings?  Why? 

W^hat  is  the  effect  of  loading  the  motor  ? 

Suggestions.  —  The  rheostats  that  have  been  used  in  this 
experiment  are  replaced  in  practical  work  by  a  single  motor 
starting  box  which  has  two  rheostats  'in.  it,  one  for  the 
armature  and  one  for  the  shunt  field.  In  this  starting  box 
the  first  contact  sends  the  current  through  the  shunt  field 
coil  before  it  is  sent  through  the  armature.  Do  you  see 
the  reason  why  ? 

An  interesting  experiment  is  to  run  the  motor  at  full  speed 
and  then  suddenly  cut  off  the  feeding  current  from  the  armature 
while  it  is  left  on  the  field  coil.  Watch  the  voltmeter  from 
the  time  the  current  is  cut  off  until  the  motor  stops  and  notice 
the  effect  of  the  "back  electro-motive  force"  of  the  motor. 

This  description  of  the  experimenting  to  be  done  with 
the  dynamo  and  motor  may  look  rather  formidable.  Follow 


92 


ELECTRICITY 


it  out  step  by  step;  do  as  much  as  you  can,  and  you  will 
find  that  each  step  is  simple  and  that  the  more  you  do  the 
clearer  will  be  your  understanding  of  the  action  of  these  two 
machines. 

EXPERIMENT   52 
Study  of  an  Electric  Bell.     El.  of  Phys.,  p.  363 

(a)   A  simple  circuit. 

Apparatus.  —  Electric  bell ;  push  button ;  cell ;  connecting 
wires. 

Method.  —  Couple  the  apparatus  in  series.  Study  the  con- 
nections. Find  what  adjustments  give  the  best  results  and 
trace  the  path  of  the  current. 

Conclusions. — Make  a  drawing  showing  the  connections. 
How  would  you  modify  the  connections  to  make  a  single- 
stroke  bell  ?  Experimentally  prove 
your  answer. 


(&)  Two  bells  rung  from  a 
single  push  button. 

Apparatus.  —  Cells  ;  two  bells  ;  a 
push  button;  line  and  connecting 
wires. 

Method.  —  Connect  the  two  bells 
—  which  should  be  of  a  different 
rate  of  stroke  —  first  in  series  and 
then  in  parallel,  in  series  with  the 
cells  and  push  button. 

Conclusion.  —  Make  a  drawing  of 
the  connections  used  and  state 


89 

A  B 


A'JB' 


K 


FIG.  58. 


which  method  of  coupling  up  the  bells,  series  or  parallel,  gives 
the  better  results.     Explain  why. 

(c)   A  house  bell  circuit. 

In  Fig.  58,  the  push  button  F,  at  the  front  door  of  the  house, 


THE    ELECTRIC   TELEGRAPH  93 

rings  two  bells,  A  in  the  third  floor  and  A'  in  the  kitchen.  The 
push  button  K,  at  the  kitchen  door,  rings  the  two  bells 
B  and  B'.  Make  a  drawing  of  the  wiring,  the  battery  being  in 
the  basement  at  C.  After  working  out  the  connections  in  the 
drawing,  couple  up  the  apparatus  and  test  your  plan. 

EXPERIMENT   53 
The  Electric  Telegraph.     El.  of  Phys.,  pp.  363-366 

(a)   A  short-line  telegraph. 

Apparatus.  —  A  telegraphic  outfit  consisting  of 'two  cells, 
line  and  connecting  wires,  two  keys,  two  sounders. 

Method.  —  Connect  the  apparatus  in  series.  Examine 
the  construction  of  key  and  sounder.  Test  the  core  of  the 
sounder  electro-magnet  for  magnetism,  both  when  the  key 
is  open  and  when  it  is  shut.  Operate  the  circuit  from  each 
key. 

Conclusion.  —  Make  a  drawing  showing  the  connections. 
Trace  the  current  in  its  path  through  the  circuit  and  explain 
the  reason  for  the  action  of  each  part  of  the  outfit.  How  does 
the  sounder  differ  from  an  electric  bell  ? 

(6)   A  long-distance  telegraph  line. 

Since  the  line  is  long,  its  resistance  will  be  large,  and  it  is 
necessary  to  use  that  form  of  electro-magnet  called  a  relay. 
This  has  a  high  resistance  and  its  function  is  to  close  and  open 
a  local  circuit  containing  a  battery  and  sounder. 

Apparatus.  —  A  telegraph  outfit  consisting  of  several  cells, 
two  relays,  each  of  150  ohms  resistance,  a  100-ohm  coil,  two 
keys,  two  local  circuits  consisting  of  cell,  sounder  having  a  re- 
sistance of  1.5  ohms,  and  connecting  wires. 

Method.  —  Connect  up  the  apparatus  somewhat  as  in  Fig.  59. 
Study  the  connection  and  trace  the  current  when  the  keys  are 
closed.  The  satisfactory  working  of  the  apparatus  will  depend 


94 


ELECTRICITY 


upon  the  proper  adjustment  of  the  parts  of  the   relays   and 
sounders.     Find  under  what  conditions  they  work  the  best. 


Conclusion.  — Make  a  drawing  of  the  circuit  and  describe  the 
functions  of  each  part  of  the  apparatus. 
Why  is  a  relay  necessary  ? 
Why  is  it  desirable  to  have  a  sounder  ? 


EXPERIMENT   54 

Intensity  of  Illumination.      The  Bunsen  Photometer. 

El.  of  Phys.,  p.  384 

(a)  To  find  the  relation  between  the  intensity  of  il- 
lumination and  the  distance. 

Apparatus.  —  Lamp;  scale;  two  screens,  each  1  ft.  square; 
cardboard  disk  1  in.  in  diameter. 

Method.  — Mount  the  screens  and  disk  so  that  the  centers 
of  all  shall  be  in  a  horizontal  line.  Make  a  small  round  hole 
in  the  middle  of  one  screen.  Place  the  lamp  close  to  the 
perforated  screen,  and  the  disk  at  a  distance  of  1  ft.  from 
the  screen  (Fig.  60).  Place  the  second  screen  2  ft.  from 

A  B 


FIG.  60. 

the  source  of  light  as  shown  at  A  in  the  figure,  and  measure 
the  diameter  of  the  shadow  cast  by  the  disk.  Place  the  second 
screen  at  _B,  3  ft.  from  the  source  of  light,  and  measure  the 
diameter  of  the  shadow. 

Conclusion.  —  How  do  the  diameters  of  the  shadows  compare 
with  that  of  the  disk  ?  How  do  the  areas  compare  ?  If  the 
disk  were  removed,  the  light  which  now  falls  upon  it  would 
fall  instead  upon  the  area  of  shadow,  in  each  position  of  the 

96 


96  LIGHT 

screen.  Since  the  intensity  of  illumination  is  measured  by 
the  quantity  of  light  that  falls  upon  a  unit  surface,  what  is  the 
law  of  the  intensity  of  illumination  as  shown  by  your  experi- 
ments ? 

(£»)  To  find  the  relation  between  the  intensity  of  il- 
lumination and  the  distance  by  means  of  a  Bunsen 
photometer. 

Apparatus.  —  A  Bunsen  photometer  (see  Suggestion  below)  ; 
five  paraffin  candles  ;  blocks  for  supporting  the  same. 

Method.  —  Mount  one  candle  in  the  middle  of  one  block  and 
four  side  by  side  on  the  other.  Place  the  single  candle  at  one 
end  of  the  photometer  box  and  the  four  at  the  other  end. 
Move  the  spot  box  back  and  .forth  until  a  position  is  found 
where -the  spot  disappears  when  viewed  in  the  mirrors.  Meas- 
ure the  distances  from  the  lights  to  the  middle  of  the  spot 
box.  Change  the  positions  of  the  candles  and  make  a  rede- 
termination  of  the  position  of  the  spot  box  and  a  new  measure- 
ment of  the  distances.  Repeat  with  one  candle  at  one  end 
and  two  or  three  at  the  other. 

Conclusion.  —  Are  the  distances  measured  in  accordance 
with  the  law  of  the  intensity  of  illumination  investigated  in 
(a)  ?  If  the  light  is  greater  on  the  right  side  of  the  spot  box 
than  on  the  left,  which  side  of  the  spot  appears  brighter  ? 
Why  ?  Why  does  the  spot  disappear  when  both  sides  are 
equally  illuminated  ? 

Suggestion.  —  A  Bunsen  spot  box  can  be  made  as  follows: 
In  the  middle  of  a  piece  of  unglazed  white  paper,  put  a  drop 
of  melted  paraffin.  Heat  it  over  a  lamp  until  the  spot  is  about 
half  an  inch  in  diameter  and  transparent.  Cut  a  hole  an  inch 
and  a  half  in  diameter  in  a  thick  card  three  inches  square,  and 
paste  the  paper  over  the  hole  with  the  spot  in  the  middle. 
Fix  this  in  a  vertical  position  and  arrange  two  small  mirrors 
as  shown  in  Fig.  61.  The  images  of  the  two  sides  of  the  spot 
can  then  be  observed  at  the  same  time,  and  compared  directly. 


LAW   OF  REFLECTION  FROM   PLANE   MIRRORS          97 

If  the  spot  card  and  the  mirrors  are  placed  in  a  box  with  the 
ends  and  part  of  one  side  removed,  as  in  Fig.  61,  the  apparatus 
can    be    used    in    an 
ordinary     laboratory. 
If   this    is    not   done, 
the    work   should    be 
carried  on  in  a  dark- 
ened room. 

(c)  To    determine 
the  candle  power  of  FlG-  61- 

a  j$as  or  lamp  light. 

Apparatus.  —  The  Bunsen  photometer  of  (6) ;  the  lamp  to 
be  tested. 

Method.  —  Substitute  the  lamp  for  the  four  candles  and 
determine  the  position  in  which  the  spot  disappears. 

Conclusion. — Compute  from  the  measured  distances  the 
candle  power  of  the  lamp,  assuming  that  the  candles  used  are 
standard. 

Suggestion.  —  Extend  the  measurement  to  incandescent 
lamps  if  convenient. 


EXPEKIMENT   55 

Law  of  .Reflection  from  Plane  Mirrors.     El.  of  Phys., 

p.  387 

Apparatus.  —  A  large  sheet  of  paper ;  a  piece  of  ordinary 
mirror  with  one  straight  edge  or  a  similar  piece  of  plate  glass 
with  one  side  painted  black ;  a  straight-edged  ruler ;  pins. 

Method.  —  Pin  the  sheet  of  paper  to  the  top  of  a  table 
and  draw  a  straight  line  AB  across  the  middle  of  it  (Fig.  62). 
Stand  the  mirror  upon  its  edge  with  the  reflecting  surface 
directly  over  the  line.  In  an  ordinary  mirror  the  reflecting 
surface  is  the  back  of  the  mirror ;  in  the  plate  glass  blacked 

PHYS.    LAH.    HOOK 7 


98  LIGHT 

on  one  side   it   is  the   front   surface  of   the    glass.     Stick   a 
pin  vertically  in    the   paper   about   10   cm.    in    front   of   the 

mirror,  as  at  P  in  the  figure. 
Place  the  eye  at  the  edge  of 
the  paper,  as  at  E,  and 
stick  a  second  pin  at  P'  in 
line  with  the  image  of  P. 
Stick  a  third  pin  at  P"  also 
in  the  line  of  the  image. 
Eemove  the  mirror  and 


E 

FlG  62.  draw  a  line  through  PP" 

until  it  strikes  the  line 

AB  at  C.  Draw  a  perpendicular  CD  to  the  line  AB.  Draw 
the  incident  ray  PC  and  measure  the  angle  of  incidence  PCD 
and  the  angle  of  reflection  DC  P.  Repeat  for  different  posi- 
tions of  P. 

Conclusion.  —  How  does  the  angle  of  reflection  compare 
with  the  angle  of  incidence? 

Suggestion.  —  If  the  sheet  of  paper  used  is  cross-section 
paper,  the  angles  can  be  measured  with  considerable  accuracy 
by  means  of  the  small  squares. 


EXPERIMENT   56 

To  determine  the  Position  of  an  Image  in  a  Plane  Mirror. 

El.  of  Phys.,  p.  389 

(a)  The  image  of  a  point. 

Apparatus.  —  The  same  as  that  used  in  Experiment  55. 

Method.  —  Draw  a  line  AB  (Fig.  63)  and  place  the  edge  of 
the  mirror  on  it  as  in  Experiment  55.  Stick  a  pin  vertically  at 
P  and  sight  across  it  to  its  image,  moving  the  eye  to  position  E, 
such  that  the  eye,  P,  and  the  image  of  P  will  be  in  a  straight 
line.  Stick  another  pin  at  C  in  line  between  P  and  its  image. 
Move  the  eye  to  one  side  as  at  E'  and  stick  a  pin  P'  in  line 


AN   IMAGE   IN  A   PLANE    MIRROR 


99 


•P 

A                                      D     C          B 

/x 

P 

E 

FIG.  03. 


E 


with  the  image  of  P.     Stick  another  pin  at  the  edge  of  the 

mirror  at  D.     Remove  the  mirror  and  draw  lines  from  P  to 

C  and  from  P  to  D  and 

prolong  them  until  they 

meet  at  p.     Then  p  will 

be    the    position   of    the 

image  of  P. 

Conclusion.  — How  does 
the  distance  Cp  compare 
with  the  distance  PC? 
What  direction  has  the 
line  Pp  with  respect  to  the  line  AB  ?  State  a  rule  for  finding 
the  position  of  the  image  of  a  point  in  a  plane  mirror. 

(6)  The  image  of  an  object. 
Apparatus.  —  The  same  as  that  used  in  (a). 
Method.  —  Draw  a  triangle   CDE  (Fig.    64)    on   the  paper 
in  front  of  the  mirror.     Determine  the  position  of  the  image 
of  each  vertex  of  the  triangle,  and  call  the  images  of  (7,  D,  and 

^_____^^___ E,  respectively,  c,  d,  and 

e.  Connect  these  points, 
forming  the  triangle  cde. 
Measure  the  sides  of  both 
triangles. 

Conclusion.  —  How  does 
the    size    of    the    image 
of  the  triangle   compare 
with  the  size  of  the  tri- 
angle itself?     How  does  the  image  compare  in  position  and 
character  ? 

Suggestion.  —  Lay  one  mirror  on  a  table  and  fix  a  second 
mirror  vertically  beyond  it  so  that  the  edges  of  the  mirrors 
make  a  good  joint.  Hold  a  pencil  upright  in  front  of  the 
vertical  mirror  and  study  its  image  in  both  mirrors.  Study 
your  own  image  when  your  face  is  near  the  mirrors. 


B 


FIG.  64. 


100  LIGHT 

EXPERIMENT  57 

Spherical  Concave  Mirrors.     El.  of  Phys.,  p.  393 

(a)  To  find  the  focal  length. 

Apparatus.  —  Spherical  concave  mirror ;  a  small  ground  glass 
screen;  a  meter  stick. 

Method.  —  Place  the  mirror  in  the  sunlight,  facing  the  sun, 
and  focus  the  image  of  the  sun.  on  the  ground  glass.  To  do 
this,  move  the  screen  back  and  forth  until  the  position  is 
found  in  which  the  spot  of  light  is  smallest. 

Measure  the  distance  from  this  point  to  the  mirror. 

The  distance  between  the  mirror  and  the  screen  in  this  ex- 
periment is  the  focal  length  of  the  mirror,  and  the  position 
of  the  image  of  the  sun  is  the  principal  focus. 

Conclusion.  —  Make  a  drawing  showing  the  path  of  the  sun's 
rays  to  the  mirror,  and  construct  the  path  of  the  reflected  rays. 

(b)  To  determine  the  relation  between  the  position  of 
the  object  and  the  position  and  size  of  its  real  image. 

Apparatus.  —  Concave  mirror;  meter  stick;  ground  glass 
screen;  candle  or  gas  lamp;  light  box,  consisting  of  a  tin  box 
with  a  vertical  slit,  2  cm.  long  and  2  mm.  wide,  cut  in  the  middle 
of  one  side. 

Method. — Place  the  light  box,  with  the  lamp  inside,  at  a 
greater  distance  than  the  principal  focal  length  from  the  sur- 
face of  the  mirror.  Let  this  be  nearly  in  the  line  of  the  prin- 
cipal axis  of  the  mirror,  and  move  the  screen  back  and  forth 
until  the  image  is  clearly  outlined  upon  it. 

Measure  the  distance  of  the  screen  and  of  the  light  box  from 
the  mirror,  as  well  as  the  length  of  the  image. 

Move  the  light  box  and  the  screen  until  the  image  is  of  the 
same  size  as  the  object,  and  measure  the  distance  of  both  from 
the  mirror. 


INDEX   OF   REFRACTION 


101 


Conclusion.  —  Compute  the  focal  length  of  the  mirror  from 

the  equation  —  = 1 ,  in  which  Z>,  is  the  distance  of  the 

F      D0      Dt 

image  from  the  mirror,  Z)0the  distance  of  the  object  from  the 
mirror,  and  F  the  focal  length  of  the  mirror.  How  does  this 
computed  distance  compare  with  the  measured  distance  of  the 
image  from  the  mirror  when  the  image  was  the  same  size  as 
the  object?  Make  a  drawing  showing  the  path  of  some 
incident  and  reflected  rays  in  the  first  experiment.  Compute 
the  radius  of  curvature  of  the  mirror. 


EXPERIMENT   58 
The  Index  of  Refraction  of  Water.     El.  of  Phys.,  p.  400 

Apparatus.  —  A   six-inch  battery  jar;  the  device  shown  in 
Fig.  65,  which  is  made  as 
follows :  — 

Saw  out  a  board  FH  of 
the  general  shape  shown 
in  the  figure,  with  a  pro- 
jecting part  that  fits  into 
the  top  of  the  jar.  Run  a 
wooden  rod  through  two 
guides  G  and  G'  and 
drive  a  pin  in  the  rod 
at  A.  Mark  off  a  metric 
scale  along  the  rod,  tak- 
ing A  as  the  zero  of 
the  scale.  At  .B,  a  point 
8  cm.  from  the  middle 
line  of  the  rod,  drive 
in  a  second  pin.  Along 
the  side  of  the  board 
CD  lay  off  a  scale  with  FIG.  65. 


102 


LIGHT 


its  middle   line   8  cm.  from  B,  having   its  zero  on   a   level 
with  B. 

Method.  —  Level  the  jar  and  pour  water  into  it  until  it 
reaches  B.  Place  the  eye  at  some  point,  as  E,  and  holding  a 
pin  on  some  division  of  the  scale  FD,  as  (7,  move  the  rod 
up  or  down  until  the  point  A  seems  to  be  in  the  straight  line 
EB. 

Conclusion.  —  Since  the  horizontal  distance  of  B  from  both 
scales  is  known  when  the  board  is  made,  the  measurements 
are  enough  to  determine  the  index  of  refraction  of  water 
(water  to  air).  Do  this  for  several  positions  of  A  and  take  the 
average. 

Suggestions.  —  On  a  sheet  of  cross-section  paper,  place  the 
point  B  at  the  intersection  of  two  cross  lines  and  draw  pencil 
lines  through  these  for  axes.  Locate 
A  a  distance  equal  to  AH  (Fig.  65) 
below  B  and  8  cm.  to  the  left  of  it,  as 
in  Fig.  66,  then  locate  C  a  distance 
equal  to  DC  (Fig.  65)  above  and  8  cm. 
to  the  right  of  B.  Draw  lines  BK 
and  BP  through  these  points  from  B. 
From  B  as  a  center,  and  with  any 
convenient  radius,  draw  a  circle.  From 
the  points  in  which  the  circle  intersects 
the  lines  BK  and  BP,  drop  perpen- 
diculars to  the  vertical  axis 


FIG.  66. 


namely,  KL  and  PM.  If  the  radius  BP  is  taken  as  unity, 
then  KL  and  PM  will  be  the  sines  of  the  angles  of  incidence 
and  refraction  respectively,  and  the  index  of  refraction,  water 


to  air,  will  be  the  ratio   -  .     These  distances  may  be  found 
PM 

approximately  by  counting  the  squares  on  the  cross-section  paper. 
The  same  apparatus  can  be  used  also  to  determine  the  index 
of  refraction  of  kerosene,  or  of  a  saturated  solution  of  salt  in 
water. 


INDEX   OF   KEF U ACTION 


103 


EXPERIMENT   59 

To  determine  the  Index  of  Refraction,   Air  to   Glass. 
El.  of  Phys.,  p.  405 

Apparatus.  —  A  rectangular  block  of  plate  glass  with  polished 
edges;  a  metric  scale;  sheet  of  paper;  pins. 

Method. — Place  the  plate  of  glass  upon  the  middle  of  the 
sheet  of  paper  and  trace  its  outline  with  a  sharp-pointed  pencil 
(Fig.  67).  Suppose  this  to  be  MN  in  the  figure.  Stick  two 
pins  at  A  and  B  in  such 
positions  that  a  line  drawn 
through  them  will  make  an 
angle  of  about  45°  with  the 
side  of  the  glass  plate.  Sight 
the  two  pins  from  the  other 
side  of  the  plate  and  stick 
two  pins  C  and  D  in  such 
positions  that  all  four  pins 
seem  to  be  in  exact  line. 
Eemove  the  plate  and  draw 
the  lines  AB,  BC,  and  CD. 
At  B  construct  a  line  FE, 

perpendicular  to  the  edge  of  the  plate.  With  B  as  a  center 
describe  a  circle.  Where  this  intersects  the  lines  AB  and  BC 
drop  perpendiculars  to  FE,  namely,  GH  and  KL,  and  measure 
them.  If  BG,  the  radius  of  the  circle,  is  taken  as  unity,  then 
the  line  GH  is  the  sine  of  the  angle  of  incidence  and  the  line 
KL  is  the  sine  of  the  angle  of  refraction.  Measure  these  lines 
and  determine  the  value  of  the  index  of  refraction  from  air  to 
glass.  With  C  as  a  center  draw  a  circle  and  determine  the 
value  of  the  index  of  refraction  from  glass  to  air.  Is  there 
any  relation  between  this  value  and  the  former  one  ?  Repeat 
for  a  different  angle  of  incidence. 


FIG.  67. 


104  LIGHT 

Conclusion.  —  How  does  the  direction  of  the  ray  CD  com- 
pare with  that  of  the  ray  AB  ?  Why  ? 

How  is  a  ray  of  light  affected  on  entering  an  optically  denser 
medium  ? 

How  is  it  affected  when  it  enters  a  medium  that  is  optically 
rarer  ? 

Do  your  different  determinations  prove  that  the  index  of  re- 
fraction is  a  constant  ? 


EXPERIMENT   60 

To  trace  the  Path  of  a  Ray  of  Light  from  Air  through  a 
Triangular  Glass  Prism  and  into  the  Air  again.  The 
Angle  of  Deviation.  El.  of  Phys.,  p.  406 

Apparatus.  —  A  triangular  glass  prism;   a  sheet  of  paper; 
nieter  scale ;  pins ;  protractor. 

Method.  —  Set  the  prism  on  end  in  the  middle  of  the  sheet 
of  paper,  and  trace    its   outline  with   a   sharp-pointed  pencil 
(Fig.  68).     Stick  two  pins  in  the  paper 
as  at  A  and  B  in  such  position  that  the 
line  AB  will  make  an  angle  of  about 
45°  with  the  side  of  the  prism.     Stick 
two  pins  C  and  D  on  the  other  side  of 
FIG.  68.  the  prism   in  such  positions  that  the 

four  pins  seem  to   be   in  exactly  the 

same  straight  line.  Remove  the  prism.  Draw  perpendiculars 
to  the  sides  of  the  prism  at  B  and  (7.  Determine  the  index 
of  refraction  as  in  Experiment  59,  and  measure  the  angle  of 
deviation  AEF. 

Conclusion.  —  The  angle  of  deviation  measures  the  change  in 
the. direction  of  the  incident  ray.     Show  on  your  drawing  the 
direction  the  ray  AB  would  take  if  the  prism  were  not  there. 
In  what  direction  is  a  ray  of  light  bent  by  a  triangular  prism  ? 


CONVERGING  LENS  105 


EXPERIMENT   61 

To  determine  the  Focal  Length  of  a  Converging  Lens. 
El.  of  Phys.,  p.  409 

Apparatus.  —  Converging  lens;  ground  glass  screen;  meter 
scale. 

Method.  —  Place  the  lens  in  the  sunlight  and  focus  the  image 
of  the  sun  upon  the  ground  glass.  Adjust  the  position  of  the 
ground  glass  until  the  image  is  the  smallest  possible.  Measure 
the  distance  of  the  screen  from  the  lens.  This  is  the  focal 
length  of  the  lens. 

Conclusion.—  Make  a  drawing  showing  the  path  of  the  in- 
cident rays  before  they  enter  the  lens  and  of  the  emergent  rays 
as  they  leave  the  lens  to  form  the  image.  Place  a  match  at  the 
focus  of  the  lens.  Explain  what  takes  place. 


EXPERIMENT   62 
Conjugate  Foci  of  a  Converging  Lens.     El.  of  Phys.,  p.  410 

Apparatus.  —  Convex  lens ;  the  light  box  used  in  Experiment 
57  (b) ;  a  ground-glass  screen  with  a  centimeter  scale  marked 
on  it ;  meter  stick. 

Method.  —  Mount  the  apparatus  so  that  the  centers  of  the 
slit  in  the  light  box,  the  lens,  and  the  scale  on  the  screen  are 
in  the  same  horizontal  line.  Arrange  the  distance  between 
the  light  box  and  the  lens  and  between  the  screen  and  the  lens 
until  the  image  of  the  slit  is  of  exactly  the  same  length  as  the 
slit  itself.  Measure  the  distance  of  the  object  and  of  the  image 
from  the  lens.  Change  the  relative  distances  until  you  get  an 
image  that  is  smaller  than  the  object,  and  then  until  the  image 
is  larger  than  the  object.  Measure  both  distances  and  the  size 
of  the  image  in  each  case, 


106  LIGHT 

Conclusion.  —  What  relation  is  there  between  the  distance  of 
the  image  from  the  lens  and  the  size  of  the  image  ?  Substi- 
tute your  measurements  in  the  formula = | >  and  from 

this  find  the  focal  length.  In  this  formula  F  is  the  focal 
length,  D0  is  the  distance  of  the  object  from  the  lens,  and  Dfis 
the  distance  of  the  image  from  the  lens.  How  does  this  focal 
length  compare  with  that  obtained  in  Experiment  61  ? 

Suggestion.  —  An  optical  bench  or  support,  such  as  is  shown 
in  Fig.  69,  can  be  easily  made  and  furnishes  a  ready  means  of 


FIG. 


taking  the  measurements.  By  cutting  one  end  of  the  slit  in  the 
form  of  a  V  and  the  other  end  square,  it  will  be  possible  to  de- 
termine in  each  case  whether  the  image  is  upright  or  inverted. 

EXPERIMENT   63 
The  Study  f>f  a  Photographic  Lens.     El.  of  Phys.,  p.  435 

Apparatus.  —  A  photographic  combination  lens  composed  of 
two  single  lenses  of  different  focal  lengths ;  light  box  ;  screen  ; 
optical  bench  shown  in  Fig.  69. 

Method.  —  Support  the  lens  on  the  bench  and  determine  its 
focal  length  from  several  sets  of  measurements.  Unscrew  the 
front  lens  from  the  setting  and  determine  the  focal  length  and 
size  of  image  of  the  back  lens.  Take  out  the  back  lens,  put  the 
front  lens  back  in  the  fitting,  and  determine  its  focal  length 
and  the  size  of  the  image, 


THE  STUDY  OF  A  PHOTOGRAPHIC  LENS      107 

Conclusion. — How  do  the  three  images  compare  in  size? 
How  do  the  focal  lengths  compare  ?  Under  what  conditions 
would  you  use  each  lens  in  taking  a  picture  ? 

Suggestions.  — The  filament  of  an  incandescent  lamp  used  as 
object  gives  a  good  sharp  image  in  this  experiment.  If  a  tri- 
pod and  camera  box  are  at  hand,  the  experiment  can  be  made 
by  using  a  window  as  the  object.  Examine  the  effect  of  using 
the  two  single  lenses  and  the  combination  on  a  landscape. 

Replace  the  lens  by  a  piece  of  tin  with  a  hole,  a  millimeter 
or  less  in  diameter,  in  its  center.  Study  the  images  obtained 
on  the  ground  glass  at  different  distances  between  ground  glass 
and  pinhole.  Explain  differences  in  size  and  amount  of  illu- 
mination of  images. 

Make  a  photograph  of  a  building  with  a  distinct  outline, 
using  a  pinhole  instead  of  a  lens. 


CHEMISTRIES 

By  F.  W.  CLARKE,  Chief  Chemist  of  the  United  States 
Geological  Survey,  and  L.  M.  DENNIS,  Professor  of 
Inorganic  and  Analytical  Chemistry,  Cornell  University 


Elementary  Chemistry     .  $1.10 


Laboratory   Manual     .      .   $0.50 


THESE   two  books  are   designed   to   form  a  course  in 
chemistry  which  is  sufficient  for  the  needs  of  secondary 
schools.    The  TEXT-BOOK  is  divided  into  two  parts, 
devoted   respectively    to    inorganic    and    organic    chemistry. 
Diagrams  and  figures  are  scattered  at  intervals  throughout  the 
text  in  illustration  and  explanation  of  some  particular  experi- 
ment or  principle.    The  appendix  contains  tables  of  metric 
measures  with  English  equivalents. 

^j  Theory  and  practice,  thought  and  application,  are  logically 
kept  together,  and  each  generalization  is  made  to  follow  the 
evidence  upon  which  it  rests.  The  application  of  the  science 
to  human  affairs,  its  utility  in  modern  life,  is  also  given  its 
proper  place.  A  reasonable  number  of  experiments  are  in- 
cluded for  the  use  of  teachers  by  whom  an  organized  laboratory 
is  unobtainable.  Nearly  all  of  these  experiments  are  of  the 
simplest  character,  and  can  be  performed  with  home-made 
apparatus. 

^f  The  LABORATORY  MANUAL  contains  127  experi- 
ments, among  which  are  a  few  of  a  quantitative  character.  Full 
consideration  has  been  given  to  the  entrance  requirements  of 
the  various  colleges.  The  left  hand  pages  contain  the  experi- 
ments, while  the  right  hand  pages  are  left  blank,  to  include 
the  notes  taken  by  the  student  in  his  work.  In  order  to  aid 
and  stimulate  the  development  of  the  pupil's  powers  of  observa- 
tion, questions  have  been  introduced  under  each  experiment. 
The  directions  for  making  and  handling  the  apparatus,  and 
for  performing  the  experiments,  are  simple  and  clear,  and  are 
illustrated  by  diagrams  accurately  drawn  to  scale. 


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By  GLENN  W.  HERRICK,  B.S.A.,  Professor  ofBiology, 
Mississippi  Agricultural  and  Mechanical  College. 


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tageously in  all  schools  from  the  Atlantic  to  the  Pacific.  The 
animal  forms  selected  for  study  give  the  pupil  a  good  concep- 
tion of  a  wide  range  of  forms  and  also  a  proper  view  of 
the  extent  and  variety  of  the  animal  kingdom.  At  the  same 
time  they  have  been  chosen  for  the  purpose  of  giving  point  to 
the  modern  theories  of  development  and  animal  history. 
*}[  Each  group  is  in  turn  treated  with  a  type  form  as  a  basis, 
and  the  related  forms  are  sufficiently  numerous  to  permit  of 
the  division  of  each  group.  Provision  is  thus  made  for  a  brief 
or  an  extensive  course.  Each  group  may  be  studied  as  a  unit, 
thus  enabling  the  teacher  to  arrange  the  order  of  the  topics  to 
suit  the  needs  of  any  particular  locality,  climate,  etc.  Ample 
directions  for  field  work  are  given,  particular  attention  being 
paid  to  forms  of  economic  importance. 

*J[  In  the  laboratory  manual  a  typical  member  of  each  animal 
group  has  been  selected  for  purposes  of  illustration,  but  struc- 
tural details  have  been  reduced  to  the  minimum  in  order  to 
afford  a  clear  understanding  of  the  physiological  processes.  In 
most  cases,  however,  a  sufficient  number  of  alternative  forms 
are  given  under  each  group  to  permit  the  selection  of  a  type 
familiar  to  the  pupils  of  any  locality.  The  laboratory  direc- 
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AMERICAN     BOOK    COMPANY 


OUTLINES    OF    BOTANY 

1 1. 00 

By  ROBERT  GREENLEAF  LEAVITT,  A.M.,  of 
the  Ames  Botanical  Laboratory.  Prepared  at  the  request 
of  the  Botanical  Department  of  Harvard  University 


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great  simplicity  and  definiteness  in  presentation. 
^J  The  course  offers  a  series  of  laboratory  exercises  in  the 
morphology  and  physiology  of  phanerogams  ;  directions  for  a 
practical  study  of  typical  cryptogams,  representing  the  chief 
groups  from  the  lowest  to  the  highest  ;  and  a  substantial 
body  of  information  regarding  the  forms,  activities,  and  re- 
lationships of  plants  and  supplementing  the  laboratory  studies. 
*d  The  work  begins  with  the  study  of  phanerogams,  taking 
up  in  the  order  the  seed,  bud,  root,  stem,  leaf,  flower,  and 
fruit,  and  closing  with  a  brief  but  sufficient  treatment  of 
cryptogams.  Each  of  the  main  topics  is  introduced  by  a 
chapter  of  laboratory  work,  followed  by  a  descriptive  chapter. 
Morphology  is  treated  from  the  standpoint  of  physiology  and 
ecology.  A  chapter  on  minute  structure  includes  a  discussion 
of  the  cell,  while  another  chapter  recapitulates  and  simplifies 
the  physiological  points  previously  brought  out. 
^  The  limitations  of  the  pupil,  and  the  restrictions  of  high 
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treatment  is  elementary,  yet  accurate  ;  and  the  indicated 
laboratory  work  is  simple,  but  so  designed  as  to  bring  out 
fundamental  and  typical  truths.  The  hand  lens  is  assumed 
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TEXT-BOOKS   ON   GEOLOGY 

By  JAMES  D.   DANA,  LL.D.,  late  Professor  of  Geology 
and  Mineralogy,  Yale  University 


Geological  Story  Briefly  Told $1.15 

Revised  Text-Book  of  Geology.      Fifth  Edition  (Rice).      .      .      .      1.40 
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THE  present  edition  of  the  GEOLOGICAL  STORY 
was  the  last  considerable  work  of  the  eminent  author's 
long    life.     As    geology    is   emphatically    an   outdoor 
science,  the  student  is  urged  to  study  the  quarries,  bluffs,  and 
ledges  of  rock  in  his  vicinity,  and  all  places  that  illustrate 
geological  operations.      The  prefatory  suggestions  are  full  of 
practical  help,  besides  enumerating  the  tools  and  specimens 
desirable  for  this  study. 

<U  The  REVISED  TEXT-BOOK  OF  GEOLOGY  is  here 
brought  down  to  the  present  time  as  regards  its  facts,  but  it 
still  expresses  the  views  of  its  distinguished  author.  While 
the  general  plan  and  the  distinctive  features  have  been  pre- 
served as  far  as  possible,  a  few  important  changes  have  been 
made  relating  mainly  to  zoological  and  botanical  classifications, 
to  the  bearings  of  geology  and  paleontology  upon  the  theory 
of  evolution,  and  to  metamorphism.  The  order  of  the  grand 
divisions  of  the  science,  physiographic,  structural,  dynamical 
and  historical,  remains  the  same  as  in  previous  editions. 
^J  The  present  edition  of  the  MANUAL  OF  GEOLOGY 
was  wholly  rewritten  under  the  author's  direction.  Owing 
to  the  extensive  recent  investigations,  new  principles,  new 
theories,  new  facts  relating  to  all  departments  of  the  science, 
and  widely  diverging  opinions  on  various  questions  have  all 
made  their  contributions  to  this  book.  The  later  tracing  of 
formations  and  mountain-making,  the  increased  number  of 
fossils,  the  study  of  canoris  and  other  results  of  erosion,  and 
the  development  of  petrology  are  all  prominently  treated  here. 


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